YANB even the teeniest bit U.
Indeed, if you do want to be helpful there are so many parts of "keeping an eye on a vulnerable neighbour" which would be much more appropriate for you – just the noticing that milk bottles haven't been taken in or the blinds gone up. That sort of monitoring would be expensive for them to buy in, and might be very low effort on your part.
Taxi-ing people around? There's already a profession which takes care of that: taxi drivers! This is one of the things they are for! And if your neighbour can't afford taxis then buses, patient transport, etc...
Save your energy for the intangible "noticing" that won't be needed... till it's needed.
That's if you want to do anything at all. You're absolutely not obliged to.
I say all the above as a vulnerable person myself. I'm extremely grateful when my neighbours offer to help with anything, but I absolutely do not expect it, and I sort out my own paid cleaners, gardeners, tradesmen, etc. It's not easy for me to afford at the moment, but that doesn't magically make it my neighbour's problem.