There are definite advantages to doing Stormblood while it's quiet there, and letting the crowd clear out of the new areas before you get to it.
(I'm not looking forward to having a sea of people crowded around all my quest targets...)
I don't think that's necessary though - you don't *need* the Lv70 armour until you move up into Lv70 content. You only really need to be geared to match the content you're currently doing.
Also, I'm not sure, but do the vendors unlock once the expansion is no longer current? You can access the Heavensward vendors as soon as you reach Idyllshire, but was that the case during Heavensward or did they change it after Stormblood released?
I don't think you need to worry about that. As long as you're doing all your MSQ quests with the class, and also running roulette dungeons (and/or sidequests) along the way, you should be fine.
Though of course, it's not a bad idea either, if you want to work on it before 5.0 releases.