Your vargs should be able to kill the cannons you will deal 3-4 wounds pr round of CC while he will deal a little under 2. If you don't break him (he's ld 7 loosing by 1 or 2) you will halt him from shooting another round.
I haven't read the rest of this thread, so this might have been covered already. However, keep in mind that ironblasters are CHARIOTS, and thus the beasts pulling them cannot attack you except from the front. Thus if you charge them in the flank or rear (which should be easy for vargs) you're golden.
Otherwise, I have learned the following fighting Ogres a lot:
1) Their leadership is poor. With general and BSB around they're solid, but outside that they tend to run away if you push them hard.
2) sabretusks cause panic checks. Ogre players like to forget this, but if a sabre tusk is destroyed near their ogre units or flees through them make them roll those checks.
3) a lot of ogres' hitting power comes from stomps. Things that can't be stomped (i.e. cavalry, other MI, etc.) are good options against them.
4) Never underestimate the number of wounds their little units can absorb. A unit of 9 ironguts has 27 wounds, plus characters.
5) As per above, charge ironblasters in the flank.
6) Take out their shooting! Ogre shooting is nasty.
7) Be very careful about the hellheart. Miscasting vampires will shred whatever unit they're in, leaving you vulnerable to taking a cannonball to the face. Hang back with your vampire-containing units until your vargheists have knocked out their cannons.
8) Overwhelming force: the only way to kill a gutstar is to either whittle it down first, or throw everything in your freaking army at it. Thus either throw a purple sun through it before engaging, or
chaff it while you kill everything else before hitting it with all you've got.
9) Harmonic convergence is always scarier than comet.