Indeed you can. The clear spaces requirement only affects the space in between the target and the attacker. The space the target is on can be a Vine Wall. Otherwise, like previously said, even ranged attacks couldn't reach units on Vine Walls, and that is permitted already.
I must say, I have a great appreciation of Sin-Sin ever since I built my IoS Cloak deck with her. Have like 10-20 games with her in it. Against PE and FK, she's awesome being able to sneak in a hit on their fragile summoner. I even used her to Charm a newly summoned thief to do a touchdown in 1 turn.
I made a deck with Theifz x10, Sin-Sin makes it work.
The Fallen Kingdom is Rising...