Means sir vegas slow motion not play smooth ..and speed limit only 400% ...Why cant set 500-1000 % speed

Velocity Envelope goes up to 1000 % and you can mix this with 400 % clip speed, thus resulting in 4000 % in just one go. Use blending for smoothing.

Two further options for advanced Smoothness:

1 Record at higher frame-rate

2 TWIXTOR

Sounds like OP wants "optical flow" style slow-motion. Me too!

In the meanwhile: 1) what @Grazie said. or 2) If you can find an old download of GoPro studio, you can render an intermediate using "Flux" . Works quite well and is free.

Yeah, optical flow slomo is definitely a feature I'd love to see added to Vegas.

Doesn't a significant portion of the smoothness of slow motion come from the way the footage is recorded? I mean if you are recording at 30 fps vs 180 fps, isn't that going to be a bigger factor in "smoothness" than what the software can do?

High shutter speed filming is the norm for non professional filming. So instead of shutter speed being 1/60th it might be 1/500 outdoors. Software can add interframes & it looks better than original as far as smoothness. If you are shooting properly & using 1/60, you can't add interframes because the software doesn't work well with blur, and also it won't look natural .

Screen caps it is useful too. twixtor is mostly used for creating slow motion out of gaming footage, but also interframes can work for something as dull as text scrolling up a screen. It was scrolling line by line & looked too jarring to me, so I converted the 30fps capture to 60fps & the interframes made the scrolling much more pleasing.

The software isn't blending frames. it's creating artificial frames where it predicts they should be, but yes you should have filmed at 180fps if you knew that's what you wanted.. but maybe you didnt.

But Bob isn't shutter speed a different issue to framerate? You may still only be shooting 50/60 fps even at a shutter speed of 1/500.