Brettonians can be a tough nut to crack, if you face an experienced player, but generally VC are one of the best equipped armies to beat them.
Grail Knights I find are the killer unit, in effect a unit of champions, immune to psychology, and will usually contain a cool magic banner/heroes dripping in virtues. You will face Grail knights, or the Brettonian player has gone mental. It’s likely to contain any magic users and at least 2 heroes which various virtues, and a BSB. This is often a Brettonian Death star unit, especially for new players. As with all Uber units, its got to kill allot of things to make its points back. Tar pit units are great here, Zombies are perfect, cheap and easily raised to keep these guy away from something useful. Banshee is fun to use on them, but they can often contain their general, so don’t expect great results. Law of metal, and pit of shades can also come in handy to try and kill them off.
Brettonian magic is average to poor. You shouldn't have to worry too much about offensive magic and you should be able to get off plenty of your own magic.
Brettonians are quick, but difficult to manoeuvre. Vulnerable to march blocking. The exception are Squires (light cav, rarely seen) and RAF units (Pegasus Knights, again I've not seen an army without them!). Most 2K Bret armies have at least 2 to 3 lances. The problem these guys face is that they go forward very well, but find it hard to change direction.
Combat is where Brets are best. They hit hard, and with a great save and ward save, (6+ goes to 5+ ward if you hit them with str 5 or above). Knights of the realm, the core Knight choice is best used against small or weak units, same to a degree with Questing Knights. Grail Kights are the killers. The Lance formation is great for all their Knight units, as it increases the number of attacks and adds rank bonus, for a 3 wide formation. Bretonian Heroes aren't bad either, BUT Vampires are better! Apart from the Grail Knight all lances suffer 2 major problems; firstly, after the first charge they lose the lance strength bonus (on the weapon, nothing to do with the Lance formation), this means if they are charged or failed to shatter a unit on the charge (much tougher to do against undead than any other army!) the Lance will struggle. The second problem is the long flanks of a Lance. If the lance is stuck in combat, and you can flank charge it, it loses its rank bonus, and it has huge flank to attack.
Men at arms are simply their for Brets to get a cheap core choice. Even your Zombies might take on these guys. Even the grail relic unit is only stubborn, and as such is still doomed against out numbering undead!
Brettonian shooting is poor. They can have a catapult, but we have dogs and bats to kill it, and their long bows are just nice easy points to gobble up. Smart players use them as a cheap table quarter holder/contester.
As such I think there are 2 units to concern yourself with, Grail Knights and the RAF. These I think are the stand out units. RAF (Pegasus Knights) are flying heavy cavalry. They are great for taking out small and weak units, have high toughness, multiple wounds, good save, lances and can FLY! Great for flanking, rear charging etc. Even better at taking on other flyers. Best answer is magic moves to get them into combat against something that will hurt them. The Varghulf is pretty nice for this. If it gets the charge in, it will slap down all but the largest of units. Banshee wail, if you can get near is good, as are ethereal, as you rarely see a hero and magic weapon in this unit and it lacks the ranks et al to kill a host by combat resolution quickly. Still a tricky unit to pin down and destroy.
Grave Guard and Black knights and their Wright King brothers are all great against Brettonians. Killing blow drives fear into the hearts of all Bret generals.
Summary:
Dance Macabre is your friend! Getting in the charge will often decide a game against Brets.
Out magic them to make sure your units are big and difficult to crumble, or can be kept fighting.
Great Weapon Guard are great can openers.
Take down the RAF by any and all means.
Bats are great, if you can avoid their RAF, to hunt down his catapult, archers and small men at arms units (dogs and small black knight units the same.)
Banshee works well against Knights of the Realm, Paladin Heroes, Questing Knights.
Once a Brettonian Lance formation is stuck in combat, try and have a unit ready to flank it. Dogs, raised Zombies et al. Magic move them into combat ASAP.
Grail Knights are you worst “Knightmare†(sorry bad pun). Your best bet is tie them up for as long as possible and keep them out of the game. Failing that, big Grave Guard unit, keep raising them at all costs and grind your enemy down. A vampire with Blood Drinker could be vital hear, but expect the Brets to try and kill your heroes quickly.
IoN is also your friend.