The negative report from a year ago may be affecting you. Remember, banks are being very fickle about giving loans and any blemish can be used an excuse to deny you.
I'm not sure we are getting the full story on the vet bill dispute. A dispute does not appear on your credit. If you failed to pay your credit card bill due to a dispute, then yes, you have a blemish for not paying your credit card bill.
When you dispute a charge, you are still obligated to pay the part of your bill that is not being investigated. You also must have an active investigation going on with the credit card company regarding the dispute. The dispute must have been made within a certain period of time, usually within 60 days of the billing statement with the disputed charge.
If you followed all these rules, then your credit card company needs to remove the negative issue from your credit. They CANNOT report you to a credit agency for failure to pay as long as their guidelines are followed.
I've disputed many charges on my credit card, and it has NEVER shown up on my credit record. One dispute dragged on and on for months and actually ended up involving the police and USPS, but I have no blemish on my credit record from it.