When editing a stylesheet the intellisense box pops up after hitting enter to go to a new line. As soon as you type the first character, it disappears. After typing the colon, the intellisense box comes back, and then again as you type the first character, it disappears.
I seem to remember this happening as well in html pages, but I am not able to make that happen anymore. Maybe you fixed it? Maybe I'm crazy?
Anyhow the stylesheet issue continues. I will be emailing a small video of the problem to your support email.
Even though you can reproduce the problem, I'll note one other thing. If I load the css file as a Miscellaneous file, intellisense works while vassist is loaded. The problem I described above only happens when I have the whole solution loaded.