Quick and obscure tip when dealing with VieleRETS. If you're copying your working sources/targets/extracts/batch_control_files over to another server and receive this type of message when executing a batch control file:
Warning: Invalid argument supplied for foreach() in /public_html/vrets/common/rets_lite/exchange.php on line 2133
Digging into that line in exchange.php will indicate it's a metadata issue. So simply be sure to copy your metadata folder over to the new server as well. I sometimes forget and rediscover the solution.