None of them!

I'm kidding a bit Technically, the first one is correct. But you should say: "Я плыву во Владивосток".

The rule is: if a word starts with a combination "в + another consonant", the preposition "в" changes to "во" (for easier pronunciation).
"Владивосток" starts with "вл-", so we say "во Владивосток" (direction), "во Владивостоке" (location).