Days ago I got my RX6900XT for 969Euro; one night later, it's 949...OK, unplug internet, uninstall GPU-driver(s) till the Windows basic adapter comes back, CCleaner system registry, and shut down. 

Replace the old RX5500XT with the new one, start PC, reconnect internet and wait for Windows recommended/forced installation. 
It's the version 30.0.13023.4001 without software UI. 
Hereafter you will install your favor driver. 
The newest is 22.5.1 (30.0.15021.1105). 
The second new one is 22.3.1 (30.0.15002.1004). 
The others are optional for gamer, profi-card or fresh chip 6x50-examine. 

I took an online benchmark as usual. 
The score is 3x better than my old card. 
For VEGAS, huge improvement is expected if not radical. 

Firstly, B636 is stable, and healthy.  
Grumbles attack the compatibility to former B550. 
Yes, it's confusing. 
I suffered several crashes at the beginning. 
Then, realized I have to do again what once done in B550: Preferences parameter, the UI, my default project, even the personal render templates must be renewed. 
Else you could meet a sudden black-out and astonish why. 
And projects out of B550? 
No answer. 

Back to RX6900XT. 
My post editing follows these presets regardless what source material onto the timeline. 
As you see, 4k60p output is hard for VEGAS, plus, FrameBlend-mode drives the preview to a big challenge. 

Why not the default DisableResample? Hm, you guys se/induced VEGAS team well-off but not me. 
I'd like my timeline in fluent streaming rather than broken jittering. Especially when those 24/25fps go 59.94. 

Fluency doesn't mean smoothie. 
I have a fixed editing surface containing 5 videotracks and 8 audios with a hidden MixingConsole.  
My first job is watching out where a motionblur needed, or a stabilization whether in subclip, an altering resample, or upscale...in short, timeline playback must keep quality and real time speed. No switch Preview/Draft. No proxy. 
Thus the Combo-Jumbo. 

In the past, my old card could just struggle up to Good/Quarter(960x540), i.e. 1/16 project resolution. 
For a reasonable decision I must interrupt the action from time to time and switch to Good/Half with dynamicRAMview. 

Superstitious or not, my dynamicRAM is dedicated in 16GB to coordinate with the RX6900XT. 
Now my preview maintains at Good/Half, the 1/4 project resolution is good enough for any decision; when RAMview needed, it's quick, the dedication reaches at least 25sec view in SDR-edit and 8sec in HDR10-edit. 

I collected almost every sample clips here in Forum or elsewhere for testing and potential solution. 
Again, B636 is stable - and my Jumbo smart. 
I want to materialize what VEGAS marketings veil. 
In conditions given above.  

Summary
Note A: Events in 50/60p and 120/360p will be disabled resampling for better performance or motionflow.  
Note B: Playback starts after timeline cache built ready i.e. after huge CPU/GPU activities calmed down. 
Note C: Each event head could lead somewhat frame drops; if it strikes for cache (typically ProRes which mostly bandwidth-hungry), give ca. 1 second pause, then continue. Remember previewing timeline is NOT watching video. 

1, clips with 8bit H264/AVC in diverse container, usally fullHD till 4k, have not a problem to play back in full speed; same also MXF, MPEG-2/IMX, XDCAM-EX, HD422/HDCAM-SR, PanasonicP2, DV-AVI. 

2, Blackmagic RAW, very impressive, even 12k goes smoothly; only have 24p clips, not clear about high fps. 

3, 8bit H265/HEVC, smooth till 4k with low fps; 50fps and above as well as bigger than 4k remarkable dropping on playback or lagging at cache, RAMview needed for decision. 

4, 10bit420 HEVC, usally in HLG-format till 4k, smooth with low fps, some drops when motion intensive; rarely comes 10bit420HDR10 onto timeline, if, RAMview needed. 

5, ProRes, as said, this intermediate only blocked down when workstaion has bottleneck in storage/bandwidth. 

6, 10bit420/422 XAVC in diverse container, Sony's old flag for sLOG-format, usally in 4k, amazingly smooth if fps logically matches project (e.g. 24p/30p/60p), else RAMview needed. 

VEGAS has an I/O-panel where attempts to deviate So4 from its problem: 
a), So4 can't decode HEVC properly, thus make a blacklist and force the old/legacy decoder drudging; if some HEVC escaped, a horror creeps then somewhere into the timeline. 
b), the old decoder doesn't know what is 10bit422-HEVC, so they come back to So4, together with the escaped 8bit-brother, and on timeline totally no go! 
c), the So4 has been developed since years, I just wonder why its performance still no breaking. 
But digging somewhat deeper, I find out it's very complicated. 

Playing around with the RX6900XT and MPC-HC, I got shocking stutter while watching 8k-videos in AV1/VP9. 
I see text clearly specified "VP9/AV1-decoding" - but without concrete statement till which resolution and how. 
Like the legacy decoder in VEGAS, it needs GPU-Acceleration instead of direct GPU-decoding, the LAV decoder needs this acceleration, too. 
Nevertheless, 8k-HDR10 stil stuttering. 

Currently, I batch prepare them in Handbrake as swap-file, but this is not the solution. 
I color correct/grade in HDR10-modus, need the original ones.