If you posted in a thread that later disappears, does your post count get rolled back when the thread is vanished?
Nope. You get to keep the count even though the post disappears.
Depends. If the thread is merely disappeared, your posts still exist and remain part of your post count. If the thread is actually deleted, your posts are deleted and your post count will decrease.
I don’t know how deleted messages here are handled; if they’re moved to a “holding area” or admin subforum, or if they’re just stricken. If they’re completely removed, your post count will decline. If thye’re moved to a hidden form, it depends on whether vBulletin is set up to count posts in those hidden mod/admin forums in your total post count. On most message boards, posts in a hidden admin-only, mod-only or “containment area” subforum usually don’t count.