It's fine to run both. I have been using Adsense and Vibrant for well over a year, no problems.
No problem with the size of Vibrant in text ads on my site as far as I can see. But the looming problem for in-text ads, it's already here actually, is that they basically don't work as intended without a mouse. Mobiles therefore are out as far in-text ads are concerned unless they come up with a solution.