You could just add 1.5385 to the current field every pay period... Just create a script that goes through each employee and adds that amount to the existing amount already in the field.
Self + 1.5385
But you might want to thing about taking it a bit further... What about making a lookup table... put your 1-3 years, your 3-6, etc... in the lookup table..
Then have your script based on how long they have been their lookup the amount of hours 1st... then add the proper amount to each emp... This way if you change the amount of vacation you can change it in one place and everyone gets the update...
Just a thought..
On the other hand, there's a very good chance that you have a table where you are already creating one new record per employee per pay period. If this is the case, a field in this table can use an auto-enter field option to look up the correct value for that pay period each time you add a new record and then a summary field (or the sum function from a related table) can report the total accrued vacation time for each employee.
Good point Phil... :)