Newbee here.
I know bits of this info can be found here and there and everywhere online but I'm not sure about my whole situation.

House: 1952 house with new room addition. Location: Southern CA.
After my remodel, I now have both 1x6 boards (original to the house) and 3/4" plywood (the new parts). It's about 50% each of the total 1000sq' floor.

For reference, I'm removing the original 2.25" Red Oak. We never really had any issues with it which I believe is a testament to our dry crawl space and narrow boards.
New Floor: I'm putting in 7.25" wide Engineered European Oak.
My question is..what should I do to prep and attach the planks? I was planning on getting a vapor barrier prior to deciding on getting such wide planks. But with such wide planks, I'm worried that only blind nails wouldn't be a safe bet. I've thought about some glue w/ the blind nails but then worry about loosing the vapor barrier.
Considering my location, the mix of subfloor, and the width of the planks...what would your suggestions be?
Thanks for any help.

I guess I should add that my original subfloor is not only 6" planks but it is NOT T&G and instead has about 3/16" gaps between them....if that helps or makes a difference.

I would prefer to glue down (full spread) those wide boards. Glue assist will work, but you may end up with noise, creaking, deflection in some area, its less stable than a full spread. The wider, longer planks will expand and contract more, and you'll be wishing you went with the full spread - in fact I'm doing repairs at a glue-assist job with that issue now; I've probably spent 400$ CDN in injection glue alone to fix minor squeaking in a few areas, not to mention the labor.

Weston, I assume that would mean that I'd need to purchase something like a 3/8 underlayment for the entire floor since I can't imagining doing a full glue down over my 1x6 plank subfloor. ??

With the mild climate and not having trouble with the solid Oak flooring, I would go with a quarter inch serpentine bead under each plank. Use a glue like Bostik Best in a sausage gun. You may want to add fasteners between the nails that hold the sub-floor down, Two nails are minimum but a third fastener in the middle of each board will give more insurance against squeaks where the sub-floor crosses the joists.

Engineered doesn't really move much at all. I float wood that wide all the time over concrete. If it were me and it's properly acclimated I'd use white silicone vapor retarder and just nail it.

Thanks everyone. It seems from here and elsewhere, that there's a lot of different ways to do the job. I suppose to some degree, I'm looking for an answer that I'm already leaning towards. For instance, I really didn't to do a full glue down. I was and am leaning towards blind nails with a silicone barrier or floating on cork (glue grooves) with the same SVS. The only floor I've ever done was bamboo float on cork and I found that incredibly simple although it was a small floor.
On another note, I've been measuring my subfloor moisture in the house for the last couple weeks (pin and pinless) and it's around 8%. But getting a good measure on the engineered oak has been difficult. The pinless doesn't seem to like the the birch ply/oak veneer make up. Even the pin meter gives me less confidence on this mixed wood. Oh well. Thanks again for all the insight.
jp

You can pin through the side of the veneer to get a reading for the white oak. If you hit a glue line in the birch ply you will get a false reading, same with being above and below the glue line. Eight percent is pretty average.