the emulator wont run

it would be better if all graphic renderers are added to the emulator so the person can choose one that best matches his/her machine

Sounds like a big waste of developers time and duplication of effort. Vulkan should provide everything that is needed.

will the vulkan option in the emulator be available for windows 8.1?

Probably yes. I can't find a source however. There should be no technical reason for it not to work since AMD made Mantle run on Windows 7 for example.

how much longer will it take for the perfected emulator to be released? a few years have already passed

Impossible to tell, anything could happen. Important people could join or leave the project. It will probably take a very long time.

how much longer will it take for the perfected emulator to be released? a few years have already passed

Impossible to tell, anything could happen. Important people could join or leave the project. It will probably take a very long time.

Wanted add a few more things.
Dolphin after 13 years still is not perfect and they're still finding bugs and inaccuracies.
RPCS3 will probably take over a decade to perfect and get close to be able to run. But all that depends on the people working on it. Also compared to most popular "big" emulators, RPCS3 has very few active developers.

Also about different backends - that's a very bad idea for an emulator still quite early into development. Remember, that commercial games started mostly only working at the start of last year.