More Info
It would seem that the Vendor is not being picked up in the [ECO_Get_Request] Variable but is present in the [ECO_Auth_Request] Variable
Comparison
[ECO_Auth_Request] => TRXTYPE=S&ACTION=S&BUTTONSOURCE=WebAssist_PHP_Cart_PRO2EC_UK&CANCELURL=http://#website#/pp_cancel.php&RETURNURL=http://#website#/pp_confirm.php&TENDER=P&PARTNER=PayPalUK&VENDOR=#vendor#&USER=#user#&PWD=#mypassword#&CURRENCY=GBP&AMT=0.09&HOSTADDRESS=payflowpro.verisign.com
[ECO_Get_Request] => TRXTYPE=S&ACTION=G&BUTTONSOURCE=WebAssist_PHP_Cart_PRO2EC_UK&TENDER=P&PARTNER=PayPalUK&VENDOR=&USER=#user#&PWD=#mypassword#&TOKEN=EC-45D19175U5497442E&HOSTADDRESS=payflowpro.verisign.com
as you can see Vendor is empty in [ECO_Get_Request]