1. Is the emblem worded correctly, or can I reduce its wordiness?

2. I modeled the second ability off of Gideon Jura. Why does Gideon's ability have the two riders and not just say "Gideon becomes a 6/6 creature UEOT"?

1. Two riders? Do you mean 1). Prevent all damage and 2). He's still a Planeswalker? The Planeswalker bit makes sense to me, if for no reason other than flavor. I suspect the 'prevent all damage' clause has a bit to do with Personal Incarnation and a bit to do with the fact that, well, that's his ultimate... he swings for 6.., a bit to do with the fact that it would seem a bit silly if two blocking creatures could knock a Planeswalker on his ass, and another bit of weirdness where, without it, Gideon might as well read "All creatures blocking Gideon gain Wither UEOT", since that probably removes loyalty counters.

   This might be an unfortunate precedence. On Gideon Jura, preventing all the damage makes sense because swinging for 6 is the best Gideon's got (and, don't get me wrong, that's very strong for a 5 drop). On this creature, however, the damage prevention don't seem as important. Vacila is changing into other creatures. If it turns into a dragon, your opponent may not be able to block it anyway. If it turns into a goblin... well... I don't see a problem with opening yourself open to a terrible trade. You chose to do that. Unfortunately, you still have two problems. One is that Gideon sets up a precedent, so you're fighting player expectations. The other is that creatures still probably remove loyalty counters in combat, and that sure does hurt.

   It might be best to get rid of both the "I'm still a Planeswalker" and "I prevent damage" to prevent that cruel interaction. Bill it as "She/He becomes the creature, but that leaves her vulnerable." A bit of a Mad Madam Mim problem. That also does a good job making this not feel like an Ultimate, since, depending on the board state, that's a better ability than Gideon's.

The "preventing all the damage" ability is very necessary on Gideon Jura and anything else that can be both a planeswalker and a creature at the same time. For precisely the reason that most people don't realise it is! If it didn't have that, damage dealt to it would work very oddly: it'd both be marked on it (counting up towards lethal damage) and remove loyalty counters. And I guarantee you at least 90% of players wouldn't get that right. So some kind of damage prevention ability or otherwise meaning combat damage doesn't get dealt to it in the usual way is pretty necessary.

None of this applies if it stops being a planeswalker, though.

Weird. My comments started with "2.", but the auto-format assumed I was making a list, indented everything, and corrected me by changing "2" to "1". I'll have to remember that in the future.

I was wondering about that.

Yick. Yeah, that seems to be standard Markdown behaviour - I've seen it in two or three different Markdown libraries I've tried. Sorry.