I think every one above has already answered how to transfer vendors. Use an LSMW which is the easiest way. It is advisable to keep the number range internal i.e. let SAP assign vendor numbers and maintain the legacy vendor number on the prev acct no field referred by others.
For the balances, we are going to transfer only the vendor open items. So, use a z program if you want to transfer a large number of items, if it is small, you can post manually. Please note that the best practice to close as many open items as possible in the legacy system itself and transfer minimal number of open items. You have to use a AP loading account for the transfer.
Regards
Sowmya