Clean the chalk off with TSP or similar solution. Follow the label directions and consider a mildew killer in with it, not a strong bleach solution but one with a releasing agent & therefore weaker.
Always prime when painting outside. It can be the best coat.
Mid sheen finish coat, probably satin or eggshell. Do the best job you Can now and it might last 25 years.
My previous experience was painting single 8 vinyl that was originally robins egg blue and after a fire not available in same color. So I bought white and lapped in in and painted with P&L aqua satin and the new white held nicely whereas the old blue buckled slightly in a few places. Not bad though, i'd do it again for sure as i was generally happy with the end result.
NO DARK COLORS. Ask the paint company for the LRV, light reflecting value to help determine how dark you can go.