I can't access sculpt vertex paint in 2.90 alpha I don't know, I downloaded it 3 times, and the sculpt vertex brush is just not there. I see all of the other sculpt mode brushes, but sculpt vertex paint is nowhere to be found.
Open user preferences

Under interface enable Develeper
Extras


After enabling the developer extras, Experimental will appear in
the menu enabling the experimental features making the options below
available (as of f319eec88186)