How do you retain the slow motion frame rate? Vegas Pro is automatically converting to the project frame rate.
Just use the normal speed adjustment, and it will take advantage of the additional frames to make it smoother?
Or is there a setting to preserve the original frame rate so it plays as it does on the iPhone?
iphone slow motion variable frame rate
First and foremost, Vegas Pro does not cope very well for many with Variable Frame Rate (VFR) from iPhones and Samsung phones amongst others. Many have experienced this - if you do a forum search re 'variable frame rate', there'll be a long history of this though not necessarily targeting slow motion. The most common recommendation over the years is to transcode VFR to Constant Frame Rate (CFR) via a utility such as the free HandBrake amongst others and then import the CFR transcode into Vegas Pro.
I would set your project frame rate to a normal value (24, 25 or 30p). Then add footage filmed at higher rates by right-clicking and "add at project frame rate" and you'll get slow motion. Otherwise if it's conformed in the phone whatever it gives you is what you get.
VFR phone footage is better converted to CFR first as Dexcon says.
@wilri001 You probably know all of this anyway. High frame rate captured in camera is #1 either left as is, SOOC, examples 100fps, 120fps, 60fps (which I think is your case) or #2 it’s conformed in camera, i.e. shot at say 100fps but saved in camera at 25fps.
The latter can be simpler in some ways, just set VP Project Properties to the example of 25fps and if required apply a velocity envelope to for example speed up playback to “normal” time, where a setting of 400%=normal.
A very nice tutorial …