So, let's go with 3-1-5-2... I take off in severe clear from O61,
contact approach and request vectors to the ILS at MHR (for multiple
approaches). I'm given a freq to squawk and a heading and altitude,
followed by "radar contact"... Am I now in the system as an IFR
flight?

Continuing on, I approach a layer of fog over top of MHR that is topped
out at 2000 feet... I'm at 4000 feet, there are no clouds within 2000
feet of me vertically, so I am still VFR...

Once I am over the top of this layer (and this is where I could use
some clarification), I am still encountering 3 miles visibility
(althought the airport is currently under IMC, and this may be where
I'm getting confused), 1000 feet above, 500 feet below and 2000 feet
vertical clearance. However, I can't see anything below me and am now
flying by reference to instruments alone. I can still maintain VFR
separation from traffic... Back to the previous question... Am now on
an IFR flight? Can this time "above the fog" be logged as "actual", or
not?

All of my practice approaches (except for those on a filed IFR flight
plan) have been done in VFR, with not a "cleared to....", but after
receiving vectors or via pilot nav, a "cleared for the approach". So
back to the previous question again, the response of "radar contact"...
Is that a statement of confirmation that indicates you are in the
system as an IFR flight (in this situation)?

I really do look at the IFR rating as a license to learn, and not a
right to go buzz around in the goo... I've got so much to learn!!! I
would be comfortable, though, planning and filing and flying a complete
IFR flight plan. It's just the impromptu stuff, like popups and
practice in actual that confuses me.

Thanks!
Todd