I would say yes. If you can't then you wouldn't even be able to shoot enemy units on a vine wall with ranged units.
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.