Argument for horizontal vs. vertical stretching. Another key point is selecting a WR that is ready to contribute instantly for Ponder's sake.
So, while the Vikings have been striving to grab a deep threat that can stretch the field at the split end spot, it's actually much more important to find receivers that can ensure the ball finds itself in the hands of Vikings instead of the opposition.
For that, the most important qualities won't be deep speed, but the ability to create large passing windows for Ponder, generating separation and maintaining possession. Receivers that operate well in a timing-oriented offense are critical because the Vikings value receptions more than a chance at big-play potential-gaining first-downs is critically important in a ball management offense.
Stretching out the defense horizontally creates more running room for Adrian Peterson, and there's a good reason that it does better for the running game than stretching a defense out downfield: it creates more difficult pursuit and tackling angles for the defense and is generally much harder to adapt to than downfield passing threats (rolling a safety over doesn't help against a receiver who masters timing and route-running, but it does against a speedy receiver).
There's some statistical support behind this approach. Big plays don't correlate nearly as well to wins as simply being successful on a passing down. In fact, at the Colts' SB Nation site—Stampede Blue—they ran a number of statistics and found that general drive success rate (the ability to get first downs) was the single best determinant of winning teams.
That doesn't mean that the Vikings don't need players who will be open deep, but that deep passing will be part of the playbook dealing with "constraint" plays and not part of the base offense. Chris Brown of Smart Football describes the constraint theory of offense, which Bill Walsh was a big advocate of:
More accomplished route runners with a wide catch radius may get more consideration (Allen, Hopkins, Woods,..) if that is on the FO's mind.
Finally, evaluating Christian Ponder is a top priority. If the receiver is not ready to contribute right away, then there's not much they can do to help the front office evaluate Christian Ponder. It will be difficult to find a receiver who can do that. In the past ten years, the top 64 picks have produced 14 receivers who have had over 800 yards from scrimmage in their rookie year (out of 79 who played in games). Of those, only four chosen between pick 23 and 52 produced 800 yards (out of 36 who played games). This doesn't include those who haven't made an impact in games, like A.J. Jenkins.