The split by operator will always give you a list of values (even if it happens to be a list of just one value, as in your case). There are a couple of ways to get what you want:
put item 1 of (firstname split by "Vital") into Lastname
Or, if you want to put several parts of a line into different variables, you can do something like this:
put aLine split by "Vital" into (firstname, lastname, more...)