Custom-defined functionals in Gaussian 09
Take a look at the IOp manual. Under IOp(3/74),


  IOp(3/74) Type of exchange and correlation potentials.
  
  
  -5 Becke3 using VWN/LYP for correlation.
  02 Lee-Yang-Parr correlation.
  05 VWN 80 (LSD) correlation.
  200 Hartree-Fock-Slater exchange (Alpha = 2/3).
  400 Becke 1988 exchange.
  
  
  So 100 is Hartree-Fock, 200 is Hartree-Fock-Slater, 205 is Local Spin Density, and 402 is BLYP.


They're unclear about it, but this means you can take the "sum" of a 1- or 2-digit number (which specifies the correlation) and a 3- or 4-digit number (which specifies the exchange) to build an exchange-correlation functional. So, IOp(3/74=-5) == IOp(3/74=402), and an identical way to define B3LYP is

IOp(3/74=402) IOp(3/76=1000002000) IOp(3/77=0720008000) IOp(3/78=0810010000)



  Does that mean, that the $E_{c}^{\text{local}}$-part
  
  
  cannot be defined,
  is always the VWN functional as in B3LYP, or
  is also the chosen correlation functional?
  


Looking at the available correlation functionals (incomplete, these are the lowest few),


01 VWN5
02 LYP
03 P81 (PL in keyword list)
04 P81 + P86
05 VWN3
06 VWN3 + P86
07 OS1
08 PW91
09 PBE
10 VSXC
11 Bc96
18 VWN5 + P86
19 LYP + VWN5 for scaling


your choices for local correlation functionals are rather limited. Either you choose a correlation functional that's purely local (VWN3, VWN5, or P81), or a non-local/gradient-corrected functional which in Gaussian all seem to have the local component as part of their definition (as in LYP), or they're forcibly combined (as in P86).