If voltage regulator is bad what will it do to your vehicle?

It will either undercharge or overcharge the battery.

If the battery is subjected to overcharge for some period of time, it boils dry

and is destroyed. If it's undercharged, then you may not have the cranking

power that you need for starting..

If the voltage regulator is so bad that there's no output from it at all, then you

drive along normally for some distance, unaware that you're driving on only

battery, and then at some point you reach the end of the battery, suddenly

have no ignition, and putter to a stop. In this case also, the battery is probably

destroyed. And they ain't cheap any more. Better to check the regulator, and

install a new one if needed.