Mimosa grass and VBW

The best time to spray weeds are when you see them (there are a few exceptions, like newly sodded lawns) so I’d spray them now.

Celsius or MSM Turf are both very effective, compatible with St Augustine, and neither have a temperature restriction, against VBW. Celsius is effective against “mimosa weed” (chamberbitter), I really don’t know how effective MSM Turf is against chamberbitter - though I have some chamberbitter, it is usually in landscape beds and I pull it by hand. The few times I sprayed chamberbitter I used Celsius. Re-spray in 3 to 4 weeks if necessary - good chance you will need to as many of your weeds may be mature.

P.S. look up chamberbitter to make sure that is what you are calling mimosa weed.