It possibly was written by the tenant!
I can't speak for VCAT, but QCAT (basically the same) generally doesn't allow legal representation in the actual hearing - and generally 'legal costs' extends only to the QCAT application fee of $50-$100.
VCAT may make a decision that as the tenants never occupied the property, and there was no real lost opportunity to the owner, the tenants may be liable for beak costs up to the value of the bond or something to that effect, rather than the unlimited liability that a normal breaking tenant may face.
Wait and see, but I can almost guarantee that they won't win.
If I was the OP I'd just take it through VCAT too...
I wouldn't suggest it to the OP, but if it was me I might respond to the letter, just to run up the tenants legal fees. A nothing letter - one that causes the tenants lawyer to charge 6m to open the letter, 6m to call the tenant and 18m to the write another letter. That's another few hundred the tenant is up for, if they write back and I write another nothing letter back that's another few hundred.