I have had a few orders rejected due my refund rate.
This was generated 'automatically' by Udimi due to a genuine vendor who only partially delivered on the clicks purchased.
...Udimi automatically then credited the amount.
How can I convince vendors to accept my future orders?
I can understand your situation. I don't think there's a way you can convince the vendors to accept your future orders.
There's one thing you can do – You can mention in your message (when you place the solo ads order)about your reason for a high refund rate. I think that could help to some extent. And then your refund rate will start lowering over time.
P.S. In which niche are you planning to run solo ads?