Can 11hours of video (7.9GB of .vob files) be written onto ONE DVD+R Dual Layer (8.5gb/240mins free space)?

It depends on how you have your files encoded, but yes. When a disc says it holds a certain number of minutes, they picked that number from using high quality video. If you're running lower res, lower quality, lower bitrates, you can certainly fit more minutes onto the same DVD. That's why a Commercial DVD can hold 9 GB completely filled, but a program like DVD Shrink can reduce the quality and file size so that the whole thing can be put on a 4.5 GB burnable DVD.