Let’s keep things simple:
VBAT kicks in when the processor loses power (3.3V). It is NOT dependent on any sleep modes whatsoever. The only purpose for VBAT is to maintain the RTC clock and a specific 4KB of RAM during the power “outage”.
AT THIS TIME, you cannot specify if a program variable is stored in this “special” 4KB space. This, however, will be implemented in the coming weeks. 