Put another vendor only if they pay for their portion.
If you can get the other contractors to pay for their portion of the ad then you would have contractors (friends) worthy of referring. Suppose, you ask your vendors to pay for a portion of your newsletter and they won't. Then, why would you advertise for them when you could use your advertising space for some special offers that benefit only you.
I don't think advertising your vendors is as interesting as offering some special offers and discounts the customer would like to read about. Write a story about jobs you did and how they changed your customer's lifestyle, or how you improved your customer's family life. Maybe a room addition brought a family closer together with family functions, or someone used one of your rooms to start a band and became famous, or used it to build a space ship. Investigate and find out how your work changes people's lives. Get people to dream about the benefits or your work.
I remember the car commercials where they kept opening and closing the car's doors. The music was very loud inside the car and when the door's were closed there was silence. Maybe, a window company could write their ad copy and tell about how quiet their windows are and how the customers can now hear things inside their home they could never hear before.
Try:
Special Discount Offer
20% off on all whater you can discount
For some reason, a 20% discount is more important to a customer than a discount with a dollar amount even when the amount is $500, or $1,000. You don't have to discount the entire job by 20%. Many of my ads say:
20% DISCOUNT ON ALL FIXTURES
This pulls in a lot of customers and I never gave a 20% discount off any fixtures because customers never buy fixtures from me. They usually buy their own, but the idea that they could get a 20% discount is attractive.
I am not a big fan for newsletters because I think the stories need a lot of value and must be interesting. I don't believe people have enough time to read more than 5% of their mail, and they will only scan the headlines. Therefore, I think brochures with the least number of word are the most effective ads.