Is it ok to drink alcohol (vodka-tonic) if I have a little headache?

Sure, most anything in moderation is okay.

i probably wouldnt

It won't harm you, but it may make your headache worse

Ok, yeah as in that it's not going to harm you. However considering hangovers commonly result and with that often a bad headache it's probably not advisable. Probably say it's a personal choice.