That depends on the track surface and how much wear / temp you are getting in the tires.
In my opinion, a tire needs to build some temperature to work best, and a tire wears to relieve heat. Excessive wear is usually a result of being too soft on duro. No wear and no temp is usually a result of being too hard of duro.
Being that Vega Yellows on dirt are generally used on very low bite track surfaces, you can usually soften them and be just fine. Some Vega tracks will need a tire punching in the high teens, low 20's, while others will be better suited around 52-54 just as they come out of the wrapper.
Likewise, heavier classes and higher HP classes will abuse tires more than lightweight plate classes, so you might find a harder duro tire working better for a stock heavy class than a jr1 class on the same race track. Tracks bite up differently throughout the night and can take a bit harder tire for longer runs (features) as well.
If it were as simple as "use this tire, with this prep, punching this duro" at any given track, everyone would be on the exact same stuff. In reality, the guy that makes the best tire choices throughout the night will be the guy to beat, and everyone else is chasing him.
