It sounds like there's something wrong in your plan. Maybe describe your planned arrangement in more detail (drawing?). There should be no way a membrane comes in contact with a UFH pipe.

I may be missing something here but... can you not just have your vapour control layer above your thermojute batts and below your omnie foilboards? The bracket fixings will penetrate it so I'd take care with that detail (I don't know how 'self-sealing' pro clima DA is?). The fiddly bit then is at the joist notches for the UFH pipes passing between them, where presumably you'd be using whichever of the various pro clima VCL tapes are appropriate for that joist-membrane interface, and I'd want to know what temperature range they're rated for there as the UFH pipes run across the notches you probably want something as a buffer between the pipes and membrane (or do they get fixed with clips there?)

That's if you even need a separate VCL. Maybe the omnie boards themselves can form your VCL (I see they're XPS core with a foil topside) again with the appropriate taping at the joist-board interfaces. This is probably the more pragmatic approach. The manufacturer may be able to advise on that but it's not a component any of them seem to mention at all on their product pages from a quick browse.

May I ask what made you choose a foil board system instead of say a spreader plate system? And why Omnie over others? I'm thinking about between joist UFH for our suspended timber floors too. Early days in my research though.

Thanks for the info on your choice, but

Posted By: modernvictorianReason for using Omnie was because their foilboard is a unique product which you can't buy elsewhere, also they were easy to deal with and provided a nice layout and installation plan upfront. They are a bit more expensive though. But not excessively.

Is it that unique? This system seems essentially the same: https://www.circoflopro.co.uk/products/circoboard
Also seen (more widely available) ProWarm boards which are similar but more basic- EPS not XPS as the insulation layer, and there's no woodboard on the underside, which I don't think is serving much purpose for your buildup is it? There may be others systems around they're just the first couple I found.

Regarding membrane over UFH, on this thread http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11845
Rex talks about using Kährs Tuplex Underlay which claims to be suitable for over UFH, albeit to then take engineered flooring.
(Incidentally, Rex mentions a few things he'd do differently, I need to give that thread a proper look myself.)

If the VCM is OK with the temperature I don't see a problem with just it laying over the top of the joists and UFH boards. Am I missing something?

If you do use the UFH boards as the VCL I'd think you'd need to seal the top of the joists as well, not just the join between the UFH boards and the joists otherwise you'll get vapour migrating down to the cold bits through the joists either all the way or just past the UFH boards then sideways into the batts. I'm assuming the blue membrane shown in the attachment to the second post is vapour open.