How safe is this power configuration?
Neither device is probably pulling the rated amount, though it might hit those ratings at peak.  If you started the vacuum and saw at the exact same time, you might trip the breaker.

Assuming the breaker isn't defective, if it hasn't tripped you're OK from a capacity point of view.  Keep in mind there might be other devices on the circuit too.

Ideally you wouldn't use an extension cord at all for this. While you probably won't run into any problems, you might consider a heavier wire gauge for the extension cord.  If you don't need all 50', you're also better off with a shorter extension cord.