If by "calling JSP' you mean the JSP that rendered the page that contains the form, that's entirely irrelevant. The JSP executed and sent the entire HTML page to the browser and is now out of the picture.
What you need to do now is to trigger an Ajax call based upon what client-side triggers need to initiate the population of the dropdown. This should be a request that only returns the data necessary to fill in the drop-down as its response -- not the entire page again. This response can be in any format that is convenient: XML or JSON being the most obvious.
So whatever servlet gets invoked by the Ajax request, will call your method that gives the data Veector (Vector? You should be using a List -- Vector is old) and converts the data into XML or JSON format for the page to use.
Since Java 1.2, the Vector has been considered rather passe and other implementors of List (such as ArrayList) are preferred. Doesn't mean you can't use it -- it's just a bit odd to see it used these days.