I have dealt with this problem with my vitz for years. Here is the solution. Please do not experiment with a vitz, ever, as it's costly to maintain.
What happened to my car was ABS light would come on and off by itself randomly and after some months it stayed on and ABS stopped working. I kept using the car for a year, just had to learn how to feather the breaks. You should never break hard in any car with or without ABS. The function of ABS is to assist you in panic and in emergency situation.
Here is possible solution for you.
[I]1. Do no go to mechanic and open up breaks, it won't do any good. Neither will changing tire pressure.
Go to someone with a scanner and get the car scanned. It will tell you which side sensor is not functioning properly. (usually by reading speed sensors, in some cases if tail light is not working or break switch, it also comes on so get those checked first)
Once you get the information about which sensor is not sending data or sending incorrect reading. You will need to get the electrical connections checked all the way from main hub (underneath the battery) to sensor.
Now the most important part.
If the sensor is busted, get it changed first (I think I got one for 1500rs from used parts market, they will be genuine). Just make sure it's correct side of the sensor as front left and right sensor have knobs for orientation.
After changing the sensor, drive the car normally for few days and see if the problem goes away.
If the problem is still there, get some time on your hands like 4-5 hours. Get some snacks and charge your mobile fully to pass time. And go to a good workshop and get the wheel hub opened to check the bearing. (You may need to have wheel alignment done after that)
ABS equipped cars have bearings with a magnetic ring. If yours is worn out, it can be replaced for 2800rs (2500 for bearing, 300 for replacement as only kharadia can do it). They are easily available in market and last for lifetime.[/I]
It seems your ABS unit is fine as when ABS light is off, it works correctly.
Never replace break pads with after market ones like korean or china. Only original. You will save a lot of headache.
It may be uncalled for but be prepared to spend around 5K (or less) and a whole day for this issue.