Think of the vectors as of simple 7 bit binary vectors! Don't be 
confused because it contains 2's and 5's instead of 0's and 1's. So get 
a standard 7 bit vector like unsigned(6 downto 0), start with the value 
0 for this vector and just increment it until 127 (all 1's). For your 
desired output signal, you look at this counting vector: each 0 is being 
output as 2, each 1 is being output as 5. That's it.