The problem is:
what happens if the VCS goes off-line for one afternoon
(or for one night, for a couple of days, for a week, ..., forever)?
Am I failing to comply with the AfferoGPLv3, unless I immediately shut
the network application down (until the VCS is back on-line) or I
immediately provide an alternative means to get the Corresponding
Source?
No such clause exists in the license. If you feel otherwise, please paste where the license reads anything similar to that.
If you're hosting a GPL licensed binary on one server, and the source on another, you're not required to take the binary down when the source goes down so long as you get the source server up (or replace it) in a reasonable amount of time.
If your free VCS service goes down for an extended period of time, just re-upload to another, and you've continually complied with your requirement to provide the modified coorespondance source.
If you're really worried about this, upload it to two different free VCS services.