Mason always has some solid LBs and DBs in the pipleline. There may not be another Zac Cunningham or JoeJuan Williams in that group, but they'll be decent SEC level players. #1 concern to me (which is the key to your Run Defense concern) is defensive line play. Guys like Odeyingbo - long/athletic - can make plays when they can get around their blocker. But all too often - especially in Q3 and Q4 - SEC offensive linemen start pushing them around and then the bad guys are running the ball at will. A flood of recruits and transfers could help here - it looks like more size, talent, and depth than VU has had in a long time - but mostly unproven talent. If this unit is significantly better than last year, that will take a lot of pressure off the defensive backfield and I think those guys will be good enough to get the job done. If the DL play is similar to last year (or Heaven help us a dropoff) then the LBs and DBs are going to get exposed, and "good enough" won't be.
I wouldn't bet against this prediction because it's the most likely scenario. The key to exceeding these expectations, if it happens, would be a greatly improved defense. Despite the losses to graduation/NFL, it's not totally unrealistic that this could happen. Mason has always had quality DBs and LBs in the pipeline. This staff is really good at spotting and developing talent at those positions. Where the Vandy D has suffered is on the D-line. There have been some of the smaller/quicker/athletic DLs who can make plays if they get free, but inevitably those type players have gotten shoved around by SEC O-lines. Not since Adam Butler became a Patriot has Mason had a legit "push 'em back" lineman. Not coincidentally, that's the last year Vanderbilt's defense lived up to Mason's "defensive genius" reputation. The (possible) bright spot is an influx of DL talent in the offseason via transfers and a much-hyped freshman who enrolled early and went through Spring practice (and did very well). Of course this is a big influx of talent that's unproven at the SEC level so it remains to be seen whether the DL will be much improved, or still the Achilles heel of the D (and the team). If the DL levels up, I think that puts the Purdue, Missouri, and UT games within reach. Surely not going to win all of those even with a much improved DL, but 2 of those three wouldn't be impossible. If the DL isn't an order of magnitude better, it might be a struggle to get the 5 wins you're predicting.