>>1064575 (OP)
With modern encoders VBR does not make sense anymore, unless you target a specific plattform with a certain limit. (eg. DVD with it's recommended 7.5Mbps)
These days you should stick to CFR, Constant Quality. When you have the end credits you barely need any bitrate, but when there is lots of fine details with high movement you need a lot. So it makes more sense to set a fixed Quality and let the Bitrate vary depending on what you need in the image. Sports videos will be big and anime small for the same "visual quality".
Handbrake is a good example of this, in that the 2 pass VBR branch got so stale, that even the VBR algorithm uses the CFR method, as stated by the devs.
So just stick with that. Encode a bunch of shit using handbrake's preview function around the standard 25-18 Quality range and determine what is enough for you. And then just use that Quality constantly, stop thinking in terms of Bitrate.
The "encoder preset" allows the x264/x265 encoder to number crunch longer for a better compression ratio btw. So if you are in need for high compression ratios, set that to higher values. Pic related. (values for h265, but roughly comparable for h264 as well)