I don't know how to use after effets and twixtor.

You can employ whatever appraoch you want. Of course you can render out a clip from Vegas just as well. it just seemed to me the AAF method might avoid extra lengthy renders. The rest is not strictly related to AE. CoDecs and such would fall under the same rules in Vegas - chosing the default uncompressed options creates huge files even when you export from V. You need to study this in a broader context. Render times are also perfectly normal. Effects processing takes time and most advanced effects are slow due to their complex internal algorithms even if they use GPU acceleration such as Twixter. You simply need to get used to it. Very few things in AE are and will ever be realtime, though I would assume in such a scenario even V would become slow, no matter how snappy it is otherwise.

Mylenium

Fair enough. This sounds like a good starting point:

https://bluefx.net/tutorial/import-sony-vegas-projects-to-after-effects/

Then in AE you would pre-compose the imported "master" comp and in the resulting parent comp apply the Twixtor effect with what you deem suitable default settings for your project. Then you scrub through it step by step. Where you encounter issues you split the layer (Ctrl+Shift+D) and change the settings, making sure you have enough meat to disguise the cut. Safe for a few situations where even Twixtor may fail this should get you there 90% and look much better even with defaults without creating a single actual keyframe and only hard-setting values per segment. Depending on the situation the most important thing will be to make Twixtor analyze only in one direction, so play around with the forward/ backward settings and frame ranges on segments where ghosting appears.

Mylenium

There is really little point to even attempt an advanced workflow like retiming clips if you don't even have some basics down. Yes, you need keyframes to create variable speed setups, but you can of course just as well just slap on an effect with constant settings though the whole point of even using AE and Twixtor is lost. So with all respect, clear your mind of all your preconceptions and start by learning some fundamentals in AE. I know this all sounds rude and a-hole-ish, but there's really no way around that. AE is way too complex to intuitively understand such a thing and since it is based on old concepts and paradigms from 25 years ago it simply hasn't timeline thumbnails or any of the common conveniences of most editing tools. And I guess that's the point: As fellow forum-honcho @Rick Gerard likes to keep saying: AE is not an editing tool. That's the first thing you need to get out of your head. If you can provide a proper description of what you are trying to do, possibly bolstered by screenshots, we may be able to provide soem rough guidance, but reading the online help is seriously advised, regardless.

After Effects User Guide

Mylenium