VirtueMart does not automate Authorize.net test mode. It doesn’t read classes/payment/ps_authorize.cfg.php AN_TEST_REQUEST to see if it should send test mode to Authorize.Net. A quick fix, alter

FILE CHANGE classes/ps_checkout.php lines 24, 159, 484
line 24:
define (TESTMODE,true);

Lines 159, 484, call validate with TESTMODE rather than hard-coded “false.”
159:
if ( !$this->validate_payment_method( $d, TESTMODE )) {
484:
if (!$this->validate_payment_method($d, TESTMODE)) { //Change false to true to Let the user play with the VISA Testnumber