Most of the VISCOVIC and VISKOVIC appear to be coming through Ellis Is from the Makarska/Tucepi area of the Croatian coast.The library you mention is in an area that, prior to 1918 was part of Dalmatian Croatia.If it happens that he is from either of these two places the LDS Family History Library near you will be able to order and you can rent microfilm records for both places dating back to the 1600s!But then if it turns out that he is from these places you will not want to miss the chance to visit these beautiful resort towns on the Adriatic!
I realize Ellis Is did not open until 1892 but it none the less can be an indicator of the origin of surnames.Have you found an arrival record for "Peter"?
VISCOVIC sounds like veeschovich while VISKOVIC sounds like vesskovich.Perhaps they are the same name.During the period in the 1800s you are talking about Italy ruled much of that area and even amongst the Croatian people their they spelled their names according to Italian sp. rules.
The Croatia phone book lists NO VISCOVIC in the areas mentioned above but does list VISKOVIC.
Where did "Peter" settle?Can you tell us anything else about him?Occupation, religion, etc.
Also you may want to check out the Croatia genforum where there are a couple of messages of particular interest is an early immigrant VISKOVIC to Puerto Rico.