I just change all 4 tyres n did alignment + balancing recently. But now whenever the speed hits 100 I will feel vibration from the steering. The vibration become stronger if goes above 100. Could this due to bad alignment, balancing, tyre or other factor?

Just send it back to the shop and tell them this happens, most likely to be as mentioned below:-
-check your tyre pressure (don trust those petrol station air pump, accuracy 50/50)
-Balancing done inaccurately
-Dent on your rims?May be?

Is your car slowing down on its own when you release accelerator pedal,
that probably means your car's brake pump is stuck. Sometimes can be adjusted, sometimes cannot. Its worth checking anyway.

the usual suspect...

1. tyre - got bengkak or not, either cause by old tyre, 2nd tyre, or pothole...

2. rim - do you remember passing over huge pothole that causes loud noise? chances are you rim might not have its initial round shape anymore...

3. alignment/suspension - if none of the above then might wanna look back into this...

check ur rim got dent ornot

and sent back to the shop and redo balancing

otherwise driveshaft is the last posibility that have vibration

try to do on-wheel balancing for the front 2 tyres.

off wheel balancing sometimes can't cure high speed vibration.

had similar problem, but once done on front wheels, all perfect now.

cheap also should be rm15-20 per tyre. (2 front tyres only rm30-rm40)

Thanks for the feedbacks. I checked brake n tyre pressure no problem. Will send the car back to the shop to check. Hopefully is only balancing issue.

If its the rim, how much is a medium price rim nowdays cost? i currently using bsa. saw alot of rims out there nowdays but unsure of their quality.