Felony arrest in VT at 18 yrs old. Served probation in FL. Living in PA and want it sealed...where do I start?

The first question is whether you pled under the State (VT) law or under federal law. If the former, you start with the VT expungement process and can check out:
http://www.publicrecordcenter.com/vermont_publi...
http://lawdigest.uslegal.com/expungement-of-cri...

If the conviction was under federal law, there is no comparable expungement process, although you could seek a Presidential Pardon. See generally http://www.usdoj.gov/pardon/pardon_instructions...

Assuming you did plea under VT State law, once the VT petition has been granted, you can serve a copy on all of the other states where there are records and ask that they also remove the records in their jurisdiction since the records were from the VT case that has now been expunged/sealed. I strongly suggest you get the court documents to establish in which Court the conviction occurred, and then consult with an experienced defense lawyer admitted to practice law in VT who has dealt with the expungement of crimes committed in VT, where the sentence has been served in another state. Good luck.

DISCLAIMER
This answer is provided solely for informational purposes, for you to use as a starting point when speaking directly with a lawyer in your State. I do not practice law in your State, and this answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising. I urge you to immediately contact an experienced criminal defense lawyer admitted to practice law in your State before you make any decisions about this case.