if a research needs more than 8 days and u need v mode at ttime so u will wait another 8 days
why research cant be built ?

Cauze there could be exploits. Order fleet for 123131 hours construct time, activate Vmode and login when done ... I guess thats why it's not possible to activate Vmode while any activity is going.

Exactly what hardstylex said. Improve your IGRN and your research labs and it wont take 8 days for your research.

Why are we not allowed to research more then 1 tech at a time? it could just take longer based on amount of research going.

Don't worry about the research time. When you reach lvl 15+ WAS, you will be worrying about where to get res for the next upgrade instead. Your labs will have more idle time than working.