We do have the same requirement. But in our case, the legacy ( Current System) is not Visual Basics 6. Ours is fully a database product (Oracle).
Steps to Follow
Gathering the possible approaches and finalizing the most appropriate approach which suites best for the available data.
The client is responsible for providing clean data in any of the forms ( tables, XML, Excel) with proper relations.
The SI team will analyze the given data and notifies if any required data is not available from the provided one. If everything looks pretty much good then SI team will start developing the code and will deliver as per the commitment.
After that testing team will do a round of testing to find if any data is missing.