First comment- as this is over 4K VEGAS automatically makes a proxy file, so if you're not say at best/full you won't be seeing the original file and there won't be any GPU decoding for the proxy. Without the proxy file I get GPU decoding activity with my Intel iGPU. With NVIDIA you likely need the new build of VEGAS Pro 21.
Second, is this a conversion? Variable framerate 10-bit HDR10 HEVC isn't an ideal editing format for VEGAS.
Complete name : 6K_Video.mp4 Format : MPEG-4 Format profile : Base Media Codec ID : isom (isom/iso2/mp41) File size : 95.0 MiB Duration : 15 s 989 ms Overall bit rate : 49.8 Mb/s Frame rate : 60.000 FPS Writing application : Lavf60.3.100
Video ID : 1 Format : HEVC Format/Info : High Efficiency Video Coding Format profile : Main 10@L6@Main HDR format : SMPTE ST 2086, HDR10 compatible Codec ID : hvc1 Codec ID/Info : High Efficiency Video Coding Duration : 15 s 989 ms Bit rate : 49.8 Mb/s Width : 6 016 pixels Height : 3 384 pixels Display aspect ratio : 16:9 Frame rate mode : Variable Frame rate : 60.000 FPS Minimum frame rate : 58.978 FPS Maximum frame rate : 60.893 FPS Standard : NTSC Color space : YUV Chroma subsampling : 4:2:0 Bit depth : 10 bits Bits/(Pixel*Frame) : 0.041 Stream size : 95.0 MiB (100%) Color range : Limited Color primaries : BT.709 Transfer characteristics : BT.709 Matrix coefficients : BT.709 Mastering display color primar : Display P3 Mastering display luminance : min: 0.0050 cd/m2, max: 1000 cd/m2 Maximum Content Light Level : 1000 cd/m2 Maximum Frame-Average Light Le : 200 cd/m2 Codec configuration box : hvcC
As an fyi, with creation of Vegas automatic proxy files enabled, all of the Best and Good preview settings bypass actually using them. I usually use Preview-Auto or Preview-Full when playing which look pretty nice with 4k hevc proxies and only switch to Best-Full to play short sections for selective pre-renders or dynamic ram previews. If Preview-Full or -Auto don't perform well enough, cutting the project frame-rate in half or by a quarter can help some more... that doesn't automatically go back when rendering so don't forget to restore the project frame-rate once editing is done.
Also, if you update and you don't have an Intel igpu, which I'm guessing you don't if auto-igpu is selecting Nvidia, try out both checking and unchecking experimental-hevc. Checking it might be a quicker performer, similar to unchecking legacy-hevc in the older vp21 as you've been doing. But developers mentioned not being totally satisfied with it's stability, so save your work before rendering. If performance unchecking experimental-hevc is acceptable, that's recommended. And report your experiences after update.