I don't have any information to connect David and Isaac Nichols with James Nichols who received a New Hampshire land grant in 1761 for Lot 21 in Panton, VT. Although a James appears to crop up in the Panton records at least through April 1813, I don't believe the grantee ever lived there. A land grant in 1774 mentions James Nichols "of Salisbury.
In Salisbury records the first mention of a James Nichols (Sr. as it turns out) of Waterbury. Salisbury records also show a land entry for James and Anna Nichols of Salisbury in 1784 which corresponds with a Waterbury marriage.
I haven't had occasion to search Waterbury records so I don't konw his ancestry or any siblings but I do have a note that James Nichols md. Anna Porter there 12 June 1740 and they had a daughter Sarah in 1741 and a son James in 1748. I don't have a record of other children.
In 1782, these two children sold to Isaac Benton and Anna Nichols land in Salisbury that had belonged to their father James. James Jr. was now of Litchfield while Sarah was still in Salisbury. I don't know when James Sr. died but this date leads me to believe that at least some of the Panton entries could be for James Jr.
A number of Panton residents moved on into New York and one family I know was eventually in Michigan.