Vlookup and returning previous months data

Hello,

I am trying to see if the a # is in the current month and the previous month, if so return the value in column "S".

Data

1234 12-31-2014 18% <----- return this data
1233 12-31-2014 19%
1234 01-31-2015 20% ___ <----- Here

Do I have to use an if statement or can it all be done with a vlookup formula.

Current statement that I am using is =vlookup(g100,g2:g99,12,false) I am trying to prevent going back and locating the previous month (there are over 100k lines)

I am also using =vlookup(g100,g200:g300,12,false) to see if the same # shows up in the next month as well.

All help is greatly appreciated.