This is not an empirical assessment, but my approach has fairly much been to go with the better specification where the options have been short-listed, It's been recommended by many over the years in areas as diverse as cars, whitegoods, home tech, etc to opt for the best specification that you can afford. A clear advantage with computers is that the higher spec'd computer is more likely to be useful into the future as software develops over the years and requires more and more from your computer. Some things like RAM can usually be added on to or replaced in later years to update the computer as needed.

I can't think of a time when I've ended up regretting spending (grudgingly) those extra $$$ for the better specs whereas there have been times when I've kicked myself for going 'budget-style' just to save some $$$ at the time of purchase.

Re i7 or i9, do an online search for reviews of comparisons between the 2 processors that you are considering. There's usually heaps of reviews which often are full of test results comparing the two as well as having observations from the reviewers.

Don't ignore the AMD processor offerings. Ryzen 9 3900X / 5900X.

Something with 8 fast cores (16 threads) as minimum

I've been building my own workstations specifically to run Vegas for paid work since V 3.0 around 20 years ago... IMO, CURRENTLY the $300 I9-9900K (from Microcenter) & $400 I9-10850K are the best bang/buck CPUs right now if your PRIMARY app is Vegas. At those prices, IMO you should NOT drop down to an I7 - you can save the price difference somewhere else in your system that is easier to upgrade, like the GPU... Note that the 10850K uses the newer 1200 motherboards which typically provide more features (faster network, etc.) & some degree of upgradability for the same price as an older 1151 motherboard required for the 9900K...

There is NOTHING wrong with choosing an AMD 39xx CPU or waiting for the new 5xxx CPUs to be available, but it may be a while before Vegas is able to use their potential as well as it does Intel CPUs with built-in iGPU / Quicksync video...

Since Vegas is built on older code, it THRIVES on core clock speed not necessarily having more than 8-10 cores... Therefore my 8-core I9-9900K overclocked to 5 ghz & VEGA 64 LQ combo performs nearly as good as a more expensive 16-core AMD 3950X & Nvidia 2080ti on most of the types of 4K editing I do for daily paid work... Intel I9 CPUs benefit from their built-in Quicksync video / Intel iGPU so IMO it is like have an extra GPU and/or more cores that current apps actually know how to utilize vs. more CPU cores that mostly just idle...

On that note, recently a client with 16 hours of 4K video spanning 8 different VEG files wanted me to render-out a different intermediate and ship it to him ASAP. I was busy doing another job on a different workstation, so I literally opened ALL 8 VEGs on my 9900K & had ALL 8 render at once. The CPU hit 100%, the VEGA about 50%, intel iGPU about 15%, but it rendered everything out the same day & I had it to FedEx in time for next day delivery. Until recently, the Nvidia GPUs could not render more than 2-3 instances at once & some AMD CPUs overheat quickly if you try to overclock them like you can an Intel CPU...