Just gt letter from Md. about intent to offset federal vendor payments. What are these ?

Because the information you gave in your question was limited please understand my answer will be limited. Generally speaking when an individual receives one of these notices the alleged creditor will be seeking to receive any payments due to the alleged debtor paid from the federal government. Most commonly it is income tax returns that are intercepted. I would suggest contacting the alleged creditor and obtaining more information about the alleged debt to determine if it is valid and still collectible.