You know what? I think I might just give decocting a go. It just seems like the best way to achieve the intensity of colour without sacrificing authenticity of style - in fact, I guess it would be difficult to replicate a real Marzen without decocting.Yes, decoction will indeed add colour, couldn't tell you how much per step or at what rate, while everyone seems to do it just a little bit differently! Give it a go is my advice, that grain looks all right to start trying it, maybe even drop it to two or even one base malt. FWIW, I do a 100% FM BPils Munich Helles which has done very well at State comp level two years running, I realise Vienna Lager is just a little bit more complex due to this colour issue but the point is 100% base malt and decoction has rocked my world, for you might be worthwhile with 100% Vienna or even Munich II.
BIAB and decoction go together just as well as other methods, however I've found it is worthwhile leaving a goodly bit of room for extra boiling water infusion(s) to adjust temperature after each step, so mash in much thicker than you normally would for stock BIAB, particularly if your kettle is usually full at mashing and you'll appreciate that extra head room.
I don't see BIAB decotion as any more of a challenge than 3v decoction, my biggest obstacle is the lack of a second vessle big enough. I have a 9L Chef Inox stock pot, but using the typical 1/3 total mash volume would mean 7L and a lot of boiling over. I just haven't been able to justify buying a 20L pot (and I'm concerned that a single ply, thin-walled pot like what you get from BigW will result in excessive scorching.
I just want to double check something... my understanding is that the 1/3 formula is based on 3v systems that reserve a percentage of water for sparging, therefore the runnings taken to decoct would be a lot thicker than if you took 1/3 of a full volume BIAB mash (like you said), so I would be inclined to base the volume taken on the recipe being ran through a 3v system and then adjust temperatures accordingly.
I figure I'd take the guess work out of balancing mash/sparge volumes and temperature ramps and simply apply heat while stirring the decotion in until the next rest temperature is achieved.
Maybe I will try a BigW pot hmmm