I've written many times about the importance of redundancy and back-ups. In a recent post, I talked about an economical way to provide a back-up compressor. You can also economically procure a back-up for your central vacuum.
Much as with a compressor, a twin-pump (or twin motor) system can provide a level of redundancy and often allow you to limp by if only one pump (or motor) goes down. Such systems are generally designed for larger offices, so you may need to cut back a bit if relying on less than the full capacity of your system.
For an effective short term vacuum, you can also get by with a basic wet/dry “shop vac”, again as purchased from a local hardware or home improvement center. Most shop vacs are designed to attach to a 2” PVC pipe (a fairly common size) but can also be easily adapted to most sizes of PVC (what is most often used for dental office vacuum plumbing). A good shop vac will pull over 100 CFM and likely be adequate for as many as 3 simultaneous users. Remote on/off will be difficult so you’ll need to run it almost constantly. As they aren’t designed for this type of use, you’ll likely only get a couple days out of one, but a good unit can be purchased from $100, less than a single service call and a cheap way to get by until your main unit can be addressed.
Great for cleaning floors, a basic wet/dry vacuum can work as a back-up for your dental office central vacuum too