But, if Jacksonville really thought that Blake Bortles is going to be Peyton Manning, why would they stand pat at 3 and risk the Rams or Texans grabbing him? (or somebody else trading up into one of those spots to grab him)

So the Jaguars were so confident in Bortles that they weren’t willing to trade down and hope to still get him, but yet not confident enough to move up?

The Vikings offer was reportedly 2nd-best to one from another unnamed team (who already have an established QB), when the Browns made their offer that swung the trade. King speculates that the Vikings were asked to include a future 1st and refused, but I think the asking price might just have been the high 3rd rounder (#72) plus another pick (#96 or #108). Spielman was clearly targeting a deep draft class, saying pre-draft that he planned to turn 8 picks into 10 (which he did), so he may not have been willing to package 3 picks for 1, even if the future first was off the table. It’s since been reported that the Vikings offer was “not close” from the Eagles perspective, who didn’t want to trade down as far as #40 (possibly because they wanted Dee Ford, who then went to KC with the next pick)

Albert Breer reported that the Vikings had Manziel slightly ahead of Bridgewater on their board, both apparently rated as mid-to-late first rounders ( http://www.nfl.com/news/story/0ap2000000349723/article/teddy-bridgewater-lands-with-vikings-thanks-to-careful-qb-hunt ). So their willingness to hold fire on the trade to #22 came with a safety net: with the Browns done for the round, there were no other teams at the bottom of the 1st likely to take a QB, so their close 2nd choice would likely still be there for the taking. Which is how it worked out.

Not sure if you can compare that situation to the Jags. If they had Bortles as clearly superior to the rest of the QBs, and the only one worth taking in the top 5-10, it would make sense to lock him in at #3 rather than trading down and running the risk of someone else stealing their guy.

The difference between how the teams approached the problem is probably mostly down to their evaluations (how much better is this QB than the rest) than their draft strategy, which was dictated by how they set up their boards in the first place.