As long as the prepayment continues to go down I think we can safely assume that MSFT is using MVIS components and that is why that account is debited. Now whether or not IVAS MVIS components are included in those payments is, as you say, conjecture. No one is breaking down IVAS. However based on the US Army IVAS procurement budget posted here, there are headsets deliverable starting this year in and growing in later years, and that that will, or will not, show up in prepayment reduction.
Keep in mind that for many, currently, IVAS is a sideshow, all eyes are on MVIS's Lidar at Munich in September. s2upid, Reddit MB moderator who broke down the HL2, will be attending and reporting back.