I cant get viniki on pokemon black
First you'll need a ticket to liberty island which is only available via wi-fi (mystery gift at main menu). If you don't have it now, you've probably missed your chance, but there may be another one.
There are two ways to get Victini (Don't worry I get a load of typos =P).
1. With the liberty ticket, go to Castelia city and find the liberty pier. Speak to the boat operator. Fight through the team Plasma grunts on the island and you'll find Victini. DON'T waste your masterball, save that for Kyurem (Big Ice and Dragon type follow-up to Zekrom & Reshiram In the great crater). Take it's health down into the red and use a move that freezes or puts it to sleep. This should be fairly easy as it knows endure. Save before the battle so if you K.O. it you can try again. Once it's asleep and weakened, just keep throwing ultra balls. If you've K.O'ed it and saved, don't panic, just fly to Castelia city and back and it'll still be there. DON'T run from the battle or it won't come back.
2. In the event you've run and saved, don't have the liberty ticket, or just can't be bothered, go to the GTS (Global Trade Station) and search for Victini. There are bound to be some rip-offs, so beware who you trade with, but you should be able to find a fair deal. Don't waste any legendarys on him!
Phew... Long one. Hope this helps, if you bothered to read it all =3