I think you just need to add and filters to be "context" filters. The way Tableau implements the Top N filter is that is is computed before any regular filters are applied. So with nothing filtered it shows the Top 5, but when you filter to a state it will show an Vendors for that state which are in the global top 5. By adding a filter to context, it bumps the filter up the calculation pipeline so it takes affect before the Top 5 is computed (the filter will also go grey/brown to indicate it's not a context filter.
btw there is only one vendor for AK, but for other states (with more than 5) it will show the Top 5.
Hope that helps