Hi @Martin Treadaway
There are automated tools to convert Access to Web. One was discussed on reddit, search "Moving MS Access to Web" - it did hit the nail with admins though, but comments are still there.
.NET and VB is a bugger. Impossible to move the business logic and even so, it would not run on all different Web servers.
What I would suggest is to modernize the App paradigm. Hence, moving to something which is supported widely. And something which is proven that runs light speed faster, and can scale, etc.
My organization is still using 20 years old technology. And it is not the only one I'm sure. They will never, ever, move from it, not because they can't, but because of politics and zillion hours spent on something which can't be modernized any more without completely changing the paradigm. Web changed from dial-up connection, and it's changing rapidly.
With the above reddit mentioned tool, one can demonstrate the POC in minutes. One can convert Access with >100 tables in a single day. And the App can be packaged into a single executable file if needed, so no fiddling with the Web servers. Just run the file, and point the office Web browsers to App network IP. So deployment is literally none. Security is also a top notch. It can even support SAML. Sounds too good to be true. Well, the App developed by this 7Mb large (!) tool or platform is supporting 2000 pharmacies, hence about 15000-20000 users 24/7/365.
Hope this reply will help you a bit to make a proper decision.
Dean