How to navigate onto VOR RWY 18 approach at KDEC?
I would expect to receive radar vectors to final if I was coming from the north into Decatur, IL.   However, we do not plan on radar vectors when we flight plan because we could lose our ability to communicate with ATC.
I would fly the victor airways to AXC.  I might plan on coming in on the 029 radial (V191) to AXC.  I am hoping for a clearance for the approach by the time I get to AXC.  When I hit AXC, I would them make a right turn and proceed outbound on the 349 radial on the approach plate.
With a proper clearance, I would maintain my cleared altitude till TRACS and then descend to 3100 FT and finish the approach using the procedure turn to line up with the runway.
You are more than welcome to fly off routes provided you remain within the service volume of the navaid you are using.  For AXC the service volume is from 1000' AGL to 40NM (for low altitude flight).  There are no restrictions to the service volume in the Chart Supplements.  If you are going to do this (and ATC clears you), you will fly the OROCA as a minimum altitude.  The off-route obstacle clearance altitude (OROCA) for this quadrangle is 3200 FT MSL.  You could file BLM direct AXC and only be one degree off the final approach course.  (See AIM 5-4-7(f))
However, you are still required to cross an IAF before you start the approach.  You will fly over AXC and then turn around to fly the procedure turn... you cannot just cross TRACS and descend for the landing.
A procedure turn is always required if it is depicted on the chart with four exceptions (AIM 5-4-9)

Receiving radar vectors to final approach course
On a route marked &quot;NoPT&quot; on the approach plate
Received a clearance for a straight-in apporach from ATC
Performing a timed apporach