You can add those to the setup script:
[Files]
Source: "vcredist_x86.exe"; DestDir: {tmp}; Flags: deleteafterinstall
[Run]
Filename: {tmp}\vcredist_x86.exe; \
Parameters: "/q /passive /Q:a /c:""msiexec /q /i vcredist.msi"""; \
StatusMsg: "Installing VC++ 2008 Redistributables..."
Note that the run parameters will change slightly, if you are using a different redistributable version from 2008.