Can I rough plumb a double vanity like this?

Venting needs to be 6" above the flood rim level before going horizontal.

I also dont get the 90s on the ends…

Yes the Tee Wye on the vent is upside down.

As the guys said.

1. Bottom.of vent header needs to be a minimum of 6" Bove the Flood level rim of the fixtures.
2. You need to dump waste in to a wye & 1/8th bend or a Long Turn Tee Wye. Sanitary Tees (Tee wyes) not code or good practice.
3. The Circuit type venting you show is how we pipe commercial jobs. That's the way I would do it. Mad Dog

This is nice too and less piping & drilling. Mad Dog

Its all in the code. Follow the code, pass the inspection. This is the reason masters have to learn their trade. Get the illustrated national code book and its all there. You may think logic and reason works, but its all physics. The vent opening above the roof in freezing areas has to be large enough to prevent frost closure. They used to be small until nature told them how to it must be to work right. Just one example to not ignore the codes. By the way, people do die when plumbers fail to do it right. Especially with cross connections. But isn't that true in all trades? Many do not appreciate good craftsman who save lives by preventing hazards.

This would be my vote as well. Much simpler. From the picture it looks like the far sink is less than three stud bays from the stack, well within reach of a 1-1/2" trap arm.

@Javelin As far as installing a clean out . If you are installing a vanity lavatory in your bathroom and not a pedestal lav. as you have pictured, then a clean out is very easy to install. Just install it with a wye, with the plug on the branch or the run and then install the trap.

We have been installing them for a few years now. It helps very much with drain cleaning and is a better access point for rodding. In some areas doing this has become plumbing code. We have even installed clean outs on a vent when practical.

To eliminate the confusion about your drain piping.

Horizontal to vertical… A tee wye. So the orange fitting should be (number three.)

Vertical to horizontal… A wye and 45deg. or the Combo that you have pictured (number two.) Which means the lav drain second from the left needs a wye and 45deg. or the combo and not a sanitary tee wye.

You might want to pick up a book that shows the different types of DWV (drainage waist and vent) fittings and pipe. This will help to learn what they are called and what they are used for. If you don't do that little bit of homework, I'm afraid I might further confuse you and I don't want to do that. 😋

The plumbing code and residential code are now online. You can read them online.

If you use a plain tee for the cleanout you can go down in to the waste or up in to the vent from it.

Almost there. I want a LTTY (Long Turn T Y aka Boston T Y) picking up the lav on the right. You can't dump it to a SANITARY TY that way. Mad Dog

Does the steamer have a zone valve on the drain to blow down after each cycle?

They can discharge water near boiling, it may be too hot for the piping material. See what your code requires as the steamer installation suggests. At the least you need an indirect waste connection. This is for a steam boiler, does a steamer for a shower follow the same requirement? Certainly based on discharge temperature.

You want a safety pan under it also.,

I think it is a normally open zone valve when power is turned off or a power outage hits, the tank will discharge into the drain. Probably a rare but not unheard of event?

The indirect waste will help, and you could run a section of no-hub cast iron for that particular section. I dump boiling water down my kitchen from time to time, after making pasta :) I think most people do. I imagine a small amount of water or steam is the question.

I had a sheetmetal shop customize those drain pans.

How much protection you want or what an inspector may require are the questions.

If it dumps after every cycle that should keep the trap primed, else you want a trap primer to prevent the trap from drying out.

Dissolved solids in the water are a problem. This was a commercial electric steam boiler at a gym that we service.