Just zoom all the way in on the timeline until you can’t zoom anymore. At that point, each vertical grid line on the timeline will represent one frame.
If, however you mean can you paint on a single frame or alter single pixels of a single frame, then no. Vegas doesn't have tools for paint, rotoscoping, blemish fixing etc. Not yet anyway. I'm still hoping for a rotoscoping type plugin with basicly all those kind of tools designed for a multi frame environment.
Rummor has it that Photoshop 8 will be able to edit frames within a video clip. It's due out 3rd or 4th quarter of 2003. But that's just rumor. I have no idea if it is reliable info.
However, untill then, you might be able to copy a frame to the clip board and paste into Photoshop. Then save to a still image format and drop it back onto the timeline in the exact location above your video layer. Time consuming I know. It would be really cool if we could just paste it back into the Vegas preview window. But that would probably mess with whatever compression codec was used.
Fractal Painter can edit individual frames of AVI's. That was actuatly implemented in v4 I belive (back in 96/97). I'm not sure what version is out now, but I got version 5 on e-bay a year ago for only $30. It was perfectly legal too (which I was worried it wouldn't be!)!! But, since I don't edit in DV, I don't know if it will work withthe SF DV Codec.
If I understand what you want to do, you can do this with Paint Shop Pro. It comes with a program called Jasc Animation Shop and you can load an AVI file, paint on the frames and save it. In fact, you can do a color replace within a range and convert it to a single solid color. This can be applied to one frame or all the frames in the sequence. It’s great for correcting a poorly lit green screen background. It also has onionskinning so you can see where you painted on the previous frame. This is a must for rotoscoping. It will even preview your video so you can see how the rotoscoping is coming before you export it back out to Vegas as another AVI.
You can download a free trial of Animation Shop from Jasc’s web site. I think it might be what you’re looking for.