Removed all the "forced blocking" and pseudo-rampage gobbledegook. Am I correct in my belief that without a target available (whether your own creature or someone else's) that Siphonvine doesn't get pumped?

That's correct.

Thanks for the confirmation. I had designed basically the same card during GDS2 for Shawn Main's set as a Vampire Bat, but we correctly decided it wasn't appropriate for Common because players might get confused when there are no targets or simply discouraged if their creatures are the only available targets.

But I think this is pretty nice uncommon, sort of the older brother to Putrid Leech.

Reminds me of Drooling Groodion, but the two cards play completely different roles; that's an expensive control card where this is a cheap aggro one.

I would be amused if two of these attacked together on an empty board.