Originally Posted by
joorinainen
Even thought IC Saigon has shutdown online bookings they could have booked me via email for a stay in November.

Sure, they book you now and only later tell you not IHG any longer and we will honor your rate, but no IHG related benefits and points apply. You are welcome to earn Marriott points of course (if they joined by then).
Properties will usually honor cash rates as long as they are not closing (for refurb or so). Just an IHG points booking would not be accepted I reckon.
Is it a bit disingenuous not to tell about the brand switch, well yes, but get the booking first and worry about that later.