I did an entire feature using Vegas 7 and had no problem loading 24f footage from my Canon XL-H1.
However after we upgraded to Vegas 8.0 I tried to load some footage and got the same stuttering preview and single frame captures as you did.
I no longer had access to the old Vegas 7 software, so I reloaded a copy of Vegas 6, reregistered the MainConcept Mpeg M2t codec and viola! no problems again.
I guess it must be a codec problem in the new software.
Bill
Cinevid Productions, Ltd.