whats the best way to maintain GS if no vasi available?

If there are no VASI lights, there must be an ILS frequency assigned to that particular runway - If not, I think it must be a scenery issue - As far as I know, there is no other way to maintain GS.

plenty of small airports with no tower control have neither ILS nor VASI,
I guess keeping GS at these airports totally depends on pilots' experience.....

It's true, there are timing tricks, watching your V/S, visual judgements, etc. that all are useful in spotting your landing. I've never been good at it (or at explaining it!), but there's a trick about watching the runway threshold and aiming towards that (of course keeping an eye on your speed/descent rate), and then as you pass the threshold switching your eyes to the far end of the runway to give you a good judgement. I'm sure someone'll post the technique soon.

For me though, it's just about practice. If you know the approx. VRef speed for your plane and hold that, also holding a good glide-slope descent rate (i.e. 700 ft/sec), you can usually eye whether you'll be short or not. If you're going to be short, lower your decent rate a little with the throttle, and vice versa if you'll be coming up long.

There are many strips without ILS, VASI or PAPI lights and with a loss of electrical power the cockpit avionics would be no use. So all pilots learn to land without them as a matter of course, including dead-stick landings (loss of engine).

For a visual landing, after you've trimmed out and established a good speed for final, keep the touchdown point of the runway in the same place on the windshield and you will be descending at the right angle to land on that point. It doesn't matter what your airspeed or distance from the runway is and is applicable for all types of approach, including cross-wind.

Mimic the relative vertical movement of the chosen touchdown point with the throttle. If the runway appears to move lower on the windshield, then lower the power. If it appears to move higher, increase the power.

Use the yoke and pedals only for left and right adjustments. Control vertical movement with the throttle only. Until the flare.

For runway alignment, use the runway as you would the localizer needle. If it appears to swing to the left, you need to move to the left until it appears vertical again, and vice versa.

As you cross the threshold, reduce power to idle and flare. Shift your view to the far end of the runway for as long as you can see it. After the main gear has touched down, hold the back pressure on until the speed bleeds off and the nosewheel settles onto the runway. (Different flare techinique required for a taildragger). In a jet, don't apply reverse thrust if you're going to use it until the nosewheel is on the ground.

This technique can obviously, and easily be tranferred to the instruments for an ILS approach where you watch the needles instead of the actual runway.

Ah, I knew someone would have the technique! Thanks for that detailed lesson!

A useful rule of thumb for getting the ROD right is 5 x Groundspeed so at 120kts groundspeed ROD should be about 600fpm. This should keep you on the glide or thereabouts. This is also useful for ILS etc as once you are at the correct ROD the GS needle should stay in the centre!

The way I use it is with the length of the runway. A short runway means you're too low, such as this:
https://flyawaysimulation.com/images/11/microsoft-flight-simulator-2002/

A long runway means you're too high, such as this:
https://flyawaysimulation.com/images/10/microsoft-flight-simulator-2004/
and an extreme example:
https://flyawaysimulation.com/images/10/microsoft-flight-simulator-2004/

This is the length of a runway at proper glideslope:
https://flyawaysimulation.com/images/11/microsoft-flight-simulator-2002/

Very informative, thanks

I just remembered another way to do it, I don't know how many planes are equiped with this crazy system.

It's called Automatic Landing Guidance, ALG, the F-15E has it. You designate the beginning of the runway with your bomb site, laser, or radar, and then input the course of the runway into your HSI, then punch that in, and your onboard computer figures out the glideslope and sets up a basic localizer.

Like I said, I don't know how many planes are equiped with this, the F-15 is the closest thing to a flying rock in the air, so it may be isolated to this plane.

How to judge if you're gonna make it?
One clue is the apparent motion of objects on the ground. As you look forward-downwar at you intended spot, two opticasl phenomena are happening, bothe at the same time. In the first place, all objects are growing in apparent size, becasue they are approaching closer to your eye; the runway grow, the gangars, the trees, the field beyond the airport, everything comes at you and grows. But at the same time, most of the objects, as they "grow", also show some motion; theymove to different places in your field of vision.
Some move downward. As an example, take the airport fence. In the first stages of the final glide, it is practically in fron of you; then it comes downward in your field of vision until it finally flashes by under you. Some objects move upward in your field of vision. As an example, take some field a little way beyond the airport. At first it lies practically in front of you, and your vision encompasses it easily along with the airport itself. Toward the end of the approach, when you look at your intendes landing spot, you would have to raise your eyes quite noticeaby to see that field.

Thus the objects move. The clue that tells you wether you are going to overshoot or undershoot is this: All objects that move downward, however slightly, are going to be overshot; all objects that move upward, toward the horizon, however sightly, are going to be undershot. And the objects that remain stationary in your field of vision and just grow in apparent size--those are the objects you will hit

If the intended field is bordered by trees or pole lines, accurate judgement becomes doubly important; at the same time, a mental hazard is introduced which interferes with judgment. Approaching the filed, you see the tops of the poles projecting in the the landing field. That is, you see the top of a pole against a background of airport sufrace with tufts of grass or bare spots. Now, if during the further approach the tops of the poles are apparently growing deeper into the field, however slightly, you are not going to clear the line. If the top of the poles seem to remain fixed, relative to their background, you would clear if you hadn't a landing gear; but if the tops of the poles are aparently pulling out of the field, however slightly, you will make it.
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I hope you find this useful. It's an extract of "Stick and Rudder - An Explanation of the Art of Flying" by Wolfgang Langewiesche