HD 5770, OpenGL core profile version string: 3.3 (Core Profile) Mesa 10.1.3
Download and unpack Blender from here http://www.blender.org/download/ or
install from your distrution repository.
Exact steps to reproduce the error:
1) Open Blender and go to File -> User Preferences -> System OR press
Ctrl+Alt+U and enable VBOs
2) Go to Edit mode(press Tab) on a default cube and for clear look for both
problems(unable to paint and slowdowns) subdivide it atleast a few times(press
W -> F6 -> Number of cuts 50*).
3) Go to Weight Paint mode(press Tab -> Ctrl+Tab) and make a stroke, you won't
see anything painting and experience slowdowns.
4) Disable VBOs and repeat 2 and 3 steps and everything works and really fast
then switch on VBOs again and your paint will disappear but turning off VBOs
again will show the one you draw before.
* depending on your GPU the number might be higher. For example on my GPU 20
cuts on a cube won't show any performance decrease with VBOs on thus i am using
50 for more clear look at performance problem.
As described by many users VBOs should make everything faster not slower. And
i've checked proprietary drivers VBOs work there but i have different troubles
with them thus i am using the OSS one that has a very few non crucial broken
features.