On StackExchange sites I usually use upvote. Elsewhere - vote up. What is the better variant - upvote or vote up? Is it context-specific?
What we have here was originally just a compound verb, similar to vote in which has a question devoted to it here:
But what happened on Stack Exchange is that the common compound verb became lexicalized - instead of remaining a compound of two words, it became a single word, upvote, as both a noun and a verb. This often happens when compound phrases (which tend to be relatively long) are in very common usage. People will contract and abbreviate, and these new forms stop behaving as abbreviations, and become lexical terms themselves.
So both vote up and upvote are common here on Stack Exchange. Outside Stack Exchange, where the concept of upvotes and downvotes isn't so prevalent, using the verb upvote might be met with a raised eyebrow.