Don wrote:

I just bought a house with a large slope of vinca - looks like both
major and minor.

I live in southern California. When is the best time to fertilize
this, and what should I use?

Is there a reason why you feel you need to fertilize this plant? With a
few notable exceptions, ornamental plants happily established in the
landscape seldom need much in the way of fertilization, vinca in
particular. With most folks, the issue is how to get vinca to slow down
(or vanish entirely), rather than encouraging further growth.

If you feel you must, any kind of all-purpose fertilizer applied when
new growth begins in the spring (now in zone 8, PNW) will work.

pam - gardengal