What strength of velcro is needed to adhere a vinyl banner to an outdoor tent?

How big are the banners?
What kind of wind loading might they see?
Will they have to stay in place in case of rain?
Are all surfaces flat?
Do you want a “stick on” or “sew on” Velcro strip on the tents? (I’m presuming stick-on for the banners.)

I’d contact a manufacturing or sales rep for Velcro™ products (all over the Web) and start with the answers to those questions.

Yep, not info enough here to make any recommendations.

If you’re looking for an adhesive-backed hook and loop product, your best bet is to contact an industrial distributor who sells VELCRO® brand hook and loop fasteners. “75” adhesive works well on vinyl. For small to medium size banners, standard-grade hook and loop (known as Hook 88 and Loop 1000) should be fine. You can find authorized distributors here: http://www.velcro.com/index.php?page=authorized-distributors

If you’re looking for something you can use that can be bought at Wal-mart or another retailer, these products are formulated for use on vinyl:
http://www.velcro.com/index.php?page=consumer-products-adhesive-backed-for-plastics