Venting issues

I recently had a room addition done and part of the project was opening a pass-thru into the new room above the kitchen sink, which required moving the vent for the sink. The contractor put in a new 2" vent in the new laundry room approx. 8' from the sink vent and said it would serve both no problem. Well there is a problem in that now when the washer drains it draws the water out of the sink trap and the gases now come up through the sink drain. The drywall work is complete and painted......the contractor is no longer welcome on our property due to some serious structural issues uncovered by a new friend, in the construction trade. The structural issues have been resolved and I really do not want to have to tear out any more drywall. We are financially tapped out and cannot afford a plumber right now and do not want to ask for any more free help from my friend as he has a business to run and a family to support, so I need to be able to fix this myself. I have renovated homes for the past 20yrs, but always hired the plumbing and electrical work out, as I know my limits. Any suggestions on a fix for this problem. Thanks in advance, Michael

Do you have any pictures of the plumbing before drywall?

Unfortunately no.....I am working on a drawing to post.....I am off to work so will try to post drawing tomorrow night

You may be able to install a studor vent under the sink. That's why phishfood is asking for a picture.

did you see the plumbing? can you describe what it is they did?

the pipe that is in the wall,horziontal is called an arm
the pipe that is vertical is called a stack
how long is the arm to the kitchen sink from the stack? size?

My scanner has decided that it does not want to work for me tonight so I cannot scan my drawing in!
The main waste line was reduced to a 2" line prior to the wye at the sink waste arm. The arm off the main waste line is 1-1/2", approx 4' long to a stack straight up approx 2' to the sink trap. They extended the main waste line 6' past the sink wye with 2" and a wye at the end, with a cleanout, to the washer drain and the vent stack rises up that wall and returns back approx. 10'to where the original vent exited the roof. There is an approximate 2' drop from the roof exit to the washer vent stack. There are not any fume issues at the washer drain trap, just the kitchen sink.
Is it possible that the main waste line being reduced prior to the sink wye is why it is creating a vacuum as it goes past the sink waste arm?