They are flexible as long as there worm, I had my heat gun with me being it was cold when I did my install, heated the section they were being applied to, as well as worming the strips , the strips I used are 12" long, you can use 10" ,you need 2-strips per side, the strips can be cut down, every 3 lights there is a section for cutting them .
As for installing them without removing the vent, I cant see how your gonna get them to stick, and be straight. The link below is where I purchased mine .
http://www.ebay.com/itm/2-12-YELLOW...757031?pt=US_Car_Lighting&hash=item565ece2a67