Topping up the insulation is a good idea and fixing the additional joists across the existing ones will both spread the weight over the existing timber and reduce your thermal bridges at joists to just the crossover points.
Vinyl paint, particularly several layers, will pretty well act as a vapour control, particularly if the original coat is primed direct to the plaster and not onto a lining paper. You will, however, need to attend to all penetrations. They will normally be where wires and pipes pass through the ceiling. Foam spray could be used, followed by Blowerproof or, as you suggest, Passive Purple. I only have experience of the former.
As you are boarding the insulated space that reduces the need for a top membrane to stop wind washing but do make sure that the ends are sealed to the roof plate to prevent air blowing under the boards. See