My sequence is usually: 1. cap seatstay; 2. braze dropout side & seat cluster side; 3. braze vent hole shut.

If there is no vent hole and you are brazing to the tube's interior, the hot air will want out and might interfere with the filler flow. It's possible to braze without a vent hole (just like you do went filling the vent hole) but you have to be quick and it might leave some tiny pinholes.