FWIW, in terms of driving improvements I think that intensity is more important than "dwell time" at/near VO2max. However, to maximize the cardiovascular training effect you still need go hard long enough, or go hard often enough. The latter effect shows up in the paper Robert posted, in which the group that recovered for 120% of Tmax (total 2028 s per workout) improved less than the group that only recovered to 65% of HRmax (total 1248 s per workout), even though the work intervals (of only ~2.5 min) were identical.

Fortunately, if you have a powermeter there are analytical approaches that allow you to estimate with reasonable accuracy just how much strain you're placing on the cardiovascular system. (In fact, with the right devices you can estimate all of the components of the Fick equation.)