@mrob there's an "undocumented" list available

purchaseorders

(at least on the client side) but a better way is probably to use

search.lookupFields

for the createdfrom join. No idea why it isn't exposed as a field on the record though:

require(['N/record', 'N/search'], function(r, s) {
	
	function t() {
		console.log("load: " + r.load({
			type: 'vendorbill',
			id: 1234567
		}).getSublistValue({
			sublistId: 'purchaseorders',
			line: 0,
			fieldId: 'id'
		}));

		console.log("lookupfields: " + s.lookupFields({
			type: 'vendorbill',
			id: 1234567,
			columns: ['createdfrom']
		})['createdfrom'][0].value)
	}
	t();
})

wow, thanks shea!