looks like the form wants BKK Banks Swiftcode as well as the Intermediary ?
VG says that for outgoing , that one needs to ask BKK Bank for their intermediary NOT ask VG, so possibly there might be a difference To vs From VG ; Looks may have to attempt a call to BBL, unless it's on their website somewhere, or to confirm its accuracy
for Wire back to VG , then VG would confirm it's intermediary; the agent never stated if it was also JP MChase or not /the actual Swift
sorry I don't see how to edit posts , on Thaivisa; VG's form also wants the BKK Bank address , mine was setup at the Exchange Bldg there at Asoke ; guess I'll have to look it up (as your OP states)
lastly, they said the minimum transfer amount is $100 by phone, and $1000 via vanguard.com fwiw