MongoDB Aggregation: Counting distinct fields

I figured this out by using the $addToSet and $unwind operators.

Mongodb Aggregation count array/set size

db.collection.aggregate([
{
    $group: { _id: { account: '$account' }, vendors: { $addToSet: '$vendor'} }
},
{
    $unwind:"$vendors"
},
{
    $group: { _id: "$_id", vendorCount: { $sum:1} }
}
]);

Hope it helps someone