Ooops! The kindly Doctor corrected me.....Vadania would indeed suffer a -4 to her attacks...and the wording suggests that she may lose her abilities after-all.
I think making a druid lose her abilities for using a normal wooden shield as a bash attack is against the spirit of the rules...but losing them for using shield spikes would be perfectly fine.
That's how I would rule it, if I was a DM. Or, if I was in charage of PHB errata. Mind you, in that case, I would remove the hard restrictions on Druids altogether, and make limited multi-classing an OPTIONAL rule for Monks and Paladins, instead of allowing unfettered multi-classing as the house-rule.