You might even want to go longer in your sous vide with the tri-tip, like 8 hours. You are right about reusing the wood from the previous smoking session. It might have sapped out the moisture. It'll probably still be fine. You could add an extra ounce or two of fresh wood if you are concerned. I still think tri-tip, anything beyond medium-rare, is going to be a challenge moisture and tenderness-wise, just because certain cuts of meat like sirlion, skirt steak, hanger steak, etc, are just fussy about the relationship between doneness and tenderness. Sous vide, by adding a third variable, "time", really helps level the playing field when it comes to that though. So I look forward to your results!