There are a couple of built-in VFX's that you can try to match the softness of HVX and 5D footage. There's quick blur, radial blur, linear blur, and the more radical defocus. One of the blurs should do the trick, and it won't take a moment to to try and see.
You might try duplicating the track, use gaussian blur on one of the tracks, then reduce opacity of the other until you get the soften you want. I tried it both ways, blur on top track, and blur on bottom layer. Both seemed to work pretty well. (Of course you'll have to reduce opacity on the uppermost track.)
First, I use Vegas 8.c and so I don't know what version you use or if later version have this FX.
While I use this for more intense softening (Vaseline-on-lens type thing with the Diffusion Max preset), you might also try Magic Bullet HD Movie Looks HD: experiment with the Diffusion Max through Edge Noise Soften presets (listed in alphabetical descending order) sliding them back towards 100% blend (no-fx). Note that the higher the diffusion effect, the high ends/light hues also increase.
These may not be what you need, but then, who knows...
I always like discovering new tweaks in Vegas :o)