you use mirroring for two things
that is mainly just keep the session table in sync so sessions don't have to be rebuilt after the failover
that is for making sure persistence information is also in sync. not required for some types of persistence, where no persistence state is saved on the BIG-IP, i.e. cookie persistence.
for more information and configuration see:
https://support.f5.com/csp/article/K13478
i don't totally understand your exact questions but ill try
VIP Normally required enable connection Mirroring?
require is a big word, it helps during a failover, but do see some notes about it not working for everything, i.e. FTP, ASM module, ...
SNAT: When should i use this no clue????
if you want the connections from the SNAT address to be also mirrored to the standby box, so during a failover the impact is lower
Persistence Connections ? DO you have to enable this to mirror Persistence connections Universal and other persistence profiles Mirror Persistence check
little lost at where you now are in the configuration