Index of wills, inventories, etc. in the Office of the Secretary of State prior to 1901 New Jersey: State of New Jersey, 1912 page 765; Hunterdon County Van Syckle - John 6431J; W. & Cods.; 1864 Inv. 1864.
This is the only John VS (of any spelling) in this book, in Hunterdon County, that appears to have died in the death range that you mentioned.
Why not send to the State of New Jersey Archives for a copy of this will? They send the copies out very quickly and for only $5 (at least this was the fee last Feb. when I sent for one). The cover letter should include the information found in the above mentioned book and the name of the source book along with a $5 check. Address is: State of New Jersey Department ofState New Jersey State Archives PO Box 307 Trenton, NJ08625-0307
It may or may not be the one you are looking for, you take your chances. Good luck. Albert