Best Bet If:  Your business never changes, no one retires, you're and optimist and love a crap shoot, the firm's being acquired, you're close to retirement, the budget is $0.00 or you love playing You-Bet-Your-Career.

Best Bet If:  After you wrote your custom system embodying your firm's precise way of doing business, someone wrote the perfect package which you can adapt without customizations.

Best Bet If:  You have 2X the budget & human resources you think you'll need, little time pressure, can freeze the existing system for 1-2 years, your staff knows precisely what the existing system does and can duplicate it - or  the users are willing to part with some functionality.

Also excellent for limiting your tenure as an IT manager.

Migrate to
VB.NET

Best Bet If:  You have limited time & budget, are risk-adverse, would like to standardize platforms ASAP, need to keep modifying the current system while its being converted, would rather contract for one-time jobs, and need an incremental cutover where users and developers need minimal retraining.  -- And the application still fits your needs.

Migrate to
VB.NET, then  incrementally
re-engineer.

Best Bet If:  Same as above - except that your application needs re-engineering and you'd like to start with a working 3-tier VB.NET or ASP thin-client alternative, perhaps using CorVision-like tools to generate 85-90% of your application's code.