Still not enough information.

Did you use vb.Net to develop a Web-Frontend or is it a client based solution?

What online functionality will you provide (full functionality or just part of it)?

Do you have a detailed and profound security concept to separate the different customers as well as protect the data from any kind of intrusion/manipulation?

Be aware that allowing external access to an ERP system usually is not a recommended scenario...

I strongly recommend to separate the Web functionality from the original ERP system. Not only in a separate database on the same server but on a separate server. Harden the connection between the two as much as possible. Transfer the data to the ERP system using "bullet proof" stored procedures. Be suer you know how to eliminate the risk of SQL injection.

Always keep in mind: As soon as there's a Web site to get acces o your system, you no longer have only the customers you know of but also a ton of people interested in getting the information you stored...