The difficulty is knowing what the Opening Balance is at some point in the past. I can think of 2 things, neither of which is perfect.

1) If the "End Date" is to always be Today, then you could take "Current Balance" from the Vendor table and add/subtract the transactions since the "Start Date" to calculate the "Opening Balance". The limitation here would be that they will likely want to be able to choose any End Date. Maybe you could calculate back to both the "Start" and "End Dates"?

2) Another option would be to export the Vendor balances to a transaction table in dBASE format at the end of each month. Then you could use Crystal to access the table through a subreport, find the correct record and calculate based on that figure and subsequent transactions. I've never tried this and even though it should work, it wouldn't be my first choice!