For 300 GP, you can use Solid Reason, which grants an additional Gather attempt at a node. Combined with a node that gives +1 gathering attempts (as Levis mentioned), that brings you up to a total of six, allowing you that guaranteed HQ.

It's usually more useful to gather for quantity rather than quality, but there are several occasions when HQ is absolutely required, such as gathering for class quests or gathering HQ Umbral Ore for your Forager's Dolabra later on.

And speaking of HQ Umbral Ore, at endgame Miner you mine at Unspoiled Nodes. These nodes can be mined only once per hour - but to make up for that a bit, you can mine at them six times instead of four. That's another case where Deep Vigor can come into play.

Also, when doing Mining Leves, every node allows for six swings. If you manage to hit the first five times, it's nice to be able to guarantee the sixth (though, of course, nothing you can get in a leve has a HQ version, so you're just doing it to guarantee the gather).

Deep Vigor has its place, but you won't be using it all the time.