We currently have a traditional large vendor in place to support all our custom and SaaS applications post deployment into production. This often means one vendor build the application and then hands it off to our support vendor to run the application. This isn't always yielding the best results and we are considering alternative approaches, how are other companies managing applications (SaaS and Custom) post deployment/golive?

We prefer to stay away from custom development has it become to costly in the long run. Has you stated, knowledge retention becomes an issue fast.
We have success with a strategy buil around adopting platforms and leveraging them plain vanilla. If cutomization is necessary, we use integration platforms like SAP BTP or MSFT POwer platform.
Company knowledge is kept in house has we have a COE filled with business partners that are the keepers of all knowledge: BP's, Training manuals, Functionnal requirements, etc..

I’ve seen success with having single vendor owning full SDLC. They collaborate in design, own development, maintenance and support. This way there are less handoffs, they have more ownership in design and delivery of high quality product.