Can I file a VOD letter after a judgement has been made?

I pulled up my credit report and found on it a judgement against me for a credit card that belongs to my ex-husband. He opened the account when we were married, they list me as a co-signer but I do not believe I co-signed for this account, I was unaware it existed. My ex became delinquent on the account after we had been legally separated for at least two years, we do not live in a community property state. I was never contacted by the card company about the debt so I did not know it existed, I was also never served with papers before or after the judgement. I want to have the court vacate the judgement for improper service, I'm sure the card company cannot produce a doent with my signature on it. Can I send a validation of debt request letter now or do I have to wait for the court to reopen the case?