You don't say anything about how you start the process in your question! NSIS installers are always "blocking", for a silent installer this means you just have to wait for the child process to end.
If the parent process is also a NSIS installer you can do ExecWait '"c:\path\to\installer.exe" /S /D=C:\Foo'
or if it is a batch file you must use start "" /WAIT "c:\path\to\installer.exe" /S /D=C:\Foo