Perhaps competion for resources in the XP32 bit vs Vista 64 bit - 64 bit may use more of your ram / resources for the not Vegas work freeing up resources for Vegas to use - hard to imagine almost a 50% reduction in time required though. Seems odd. If you add your system specs to your profile it would help.....
System spec.
=======================
Asus P5KR mobo
Q6600 CPU
8 GB RAM
Vista 64 OS drive
WP 32 OS drive
nVidia 8800 GTS 512 Ge Force
WD 10000 rpm Raid-0
Source footage:
1920x1080 30 fps from 5D2
Normal workflow = transcoding to Cineform 1920x1080 - now instead using 960x540 uncompr AVIs during editing, then exchanging those for fullsize uncompr AVIs when rendering.
The 8GB of RAM could well mean that under Vista 64 all or a lot of your video is already cached compared to running under XP. With longer projects I seriously doubt you'll see anything like that speed difference.