1、LTC6803 In most applications, are the batteries under test used for direct power supply？
2、Can VREG use as a power supply for a communication isolation chip in use of SPI？
1) Generally yes since the the usage is typically either mobile or permanent. If the parts were used as a test fixture, then it might make sense to power from the host system, perhaps using dc-dc converters (crude example on pg 36). V+ need only be equal to or greater than the top cell potential and can be a fixed supply if desired (like 48V).
2) The Vreg output cannot supply a large continuous current or the IC will get too hot, though I think some isolators draw low enough power to work OK (that's the connection on pg 13). Another workaround I saw was to add an NPN follower off Vreg, with collector directly to top cell voltage, to form a 4.4V rail with more current capability. Bear in mind that Vreg is always on, so supply loading will be a continuous battery drain that may be unintended unless a shutdown function is added. Also note that there exist isolators with built-in DC-DC so that no power is taken from the 6803, but be careful as logic lows at the SPI lines can initiate unwanted daisy chain current when the system is idle, that's why there is the bus-disconnect buffer shown in the apps section pg 34.