I (and many others) use caffeine for a pre-workout "push" with good results.
Yes, it causes vasoconstriction, as well it elevates cortisol levels in the body.
If you're really concerned about that, you can take measures to somewhat counter those effects such as using a known vasodilator like arginine and anabolism-promoting nutrition such as quality proteins and aminos.
Honestly though, I really think that unless you've got some type of biological conditions/imbalances, you probably don't need to worry about caffeine significantly hampering your training results.
This is pure conjecture on my part, but if caffeine was dramatically detrimental to exercise performance, I believe that more trainers would have caught onto the fact by now.