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?

You are not IFR until you hear "N4234J is cleared to MJB via..." or the
ilk. If you got that on the ground, you were IFR ("instrument flight
RULES") from the getgo, otherwise you are still VFR until you get an
actual clearance. "Radar contact" has little or nothing to do with IFR.

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.

You are still VFR, and you are flying visually. You are =navigating= by
instruments. So long as you can maintain cloud clearances and
visibilities, you are legal to fly under VFR ("visual flight RULES").
It may be however that you are between layers, and have no horizon with
which to orient yourself. In this case you are IMC ("Instrument
meteorolgical CONDITIONS") while still legal to fly under VFR. If you
are not instrument rated, this is dumb. Even if you are instrument
rated, this could be dumb. However, it is legal. If you are just above
the fog and can control the aircraft visually, you do not log "actual".
However if you are between layers and =require= the flight instruments
to maintain control (not just navigate), then this is "actual" and
should go in the logbook as such. Similarly, over the water, at night,
with no moon and nothing to orient yourself, even though it could be
severe clear, is "actual". It's legal VFR, and loggable as actual.

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".

In that case you were operating VFR, and are required (via your safety
pilot) to maintain visibility and cloud clearances, and avoid aluminum
yourself). If you were practicing in actual conditions, you would hear
the magic words "cleared present position to WVS via ..."

Jose
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