Besides, there are two ways to look at it, in terms of the movie as a fictional construct and in terms of it as part of the putative Trek universe as we understand it today. In the latter context, if we're evaluating it in light of later "discoveries" about how the Trek universe works, we have to interpret V'Ger as employing a warp field/bubble that encompasses its cloud.
I'm OK with the retcon that the entire cloud is within a TNG-era warp field. However, I was thinking in terms of the former context to explain how things were perceived in 1979. I'd forgotten the Making of TMP memo you described (shoot, I don't think I've read it in at least 30 years), although, I do recall mention of the "surfing" part of it now. So maybe there was some idea in TMP that this is how it was working (although, I do like your idea of the cloud being some kind of a subspace effect), but it wasn't really conveyed to the special effects staff, who for the most part kept things surprisingly consistent with how they depicted Enterprise operating. Or perhaps they were going with the assumption that if Enterprise deactivated her warp drives she'd drop out of V'ger's warp bubble unless there was actual or tractor beam contact keeping them physically linked.