When does IFR begin in VFR?

Wow!
1) You are not "IFR" in controlled airspace (anything but G) until you
are told "Cleared to somewhere".
2) You must tell ATC "I'd like to pick up my IFR". They will not simply
give it to you when you call up. Most likely its not even in front of
them.
3) If you were "IFR" you CANNOT change your altitude w/o ATC oking it.
4) You said it was "VFR". There really is no such thing. It can be
"VMC" or "VFR conditions" but VFR is a set of regulatory requirements,
not weather.

So, you can be IFR in VMC, IFR in IMC or VFR in VMC. I wouldn't
recommend VFR in IMC though.

-Robert, CFI