That is an excellent question that’s not quite as easy to answer. NLE Software is VERY complex. I’ve approached Magix with similar questions, on which hardware they recommend and they don’t even know.
I have a build that cuts video quite well, but timeline playback still isn’t perfect. I custom made the machine myself. I easily have $5,000 into the computer now. I’ve been wanting to get a second graphics card to link up to my existing and complete my ultimate build and discovered they are sky high in price yet. However…
Here’s the one tannate of software. It ALWAYS lags hardware design. My system was built off old less expensive hardware. At the time was hardware that came out two and three years prior. Last generation stuff. Magix and Vegas are JUST NOW catching up with those hardware designs from 5 to 10 years ago. No joke.
I have bought nearly every vegas update in last 10 years. With each update my NLE runs faster and faster. For instance they just started supporting H.265 playback. Before it was software decoding and sucked. Now I can actually edit h.265 and play it back in the timeline without issue. I bought no hardware to do that, just updated vegas.
Long story short, I’d wait till the cards get back down in price. I’d use what I have, can you share the chipset? I’ll compare I think mine is a Radeon RX480 w/ 8GB video memory if I recall correctly. If you want to see what I built for my system you can see it here:
https://creativecow.net/forums/thread/ultimate-vegas-machineae/
It’s likely Magix doesn’t even fully support what you have in your system as of yet. Especially if it’s a newer machine.
Another thing you can do and will want to do is open your task manager. Do a timeline playback as well as render and see how it effects your system performance. I think there’s a GPU monitor in there too. It will immediately show you if Vegas is even utilizing your GPU.
You can determine what your bottleneck is there. Typically it’s the CPU as the easy / quick way to write software is to write it with the CPU, not the GPU.

Ultimate Vegas Machine?