Microvax II won't boot, hardware error?

I wouldn't recommend that. If the plastic guide is already fully seated (as it should have been when the connector was pressed on at the factory) then putting it in a vice is only likely to damage the connector. Trying to re-press ribbon cables is somewhat futile anyway. If individual IDC connections are loose, it is time to clip and replace the connector (or build a new cable if it is an inside connector). Given the nature of the IDC terminations used in these connectors, they are unlikely to fail on their own and most of the failures I've seen have been with cables that people would repeatedly unplug by pulling on the cable. In a case such as this case no amount of re-pressing the connector is going to be able to repair the damage done to the IDC terminations.