I would like to make the reaper as productive as possible. In the search for ways to achieve this goal, several questions arose. the main one is about the buffer. Since I write electronic music, I only use VSTi instruments no analog synths or guitar. So the question arises: how important is the buffer for me? The default buffer size in the reaper is 1200ms, is that good or i can afford more withour bad issues during playback VSTi through midi? Usually when we mention latency, everyone talks about its importance for fast playback of an analog instrument in a computer, as is the case for purely use VST and VSTi?

 

You are speaking about MEDIA buffer here ... not AUDIO buffer.
Audio buffer is around 10 ms and related to soundcard and ASIO driver. This one is important ... and not related to Reaper itself.

Media buffer has only impact on RAM/disk when streaming samples/audio from disk.

Read this :
https://forum.cockos.com/showthread.php?t=56079