Are clickbank vendors allowed to promote their own product?

Yes, it's allowed. (Why wouldn't it be?).

The vendor can make sales through Clickbank either without an affiliate hoplink, just by sending traffic to the sales page (these are processed as "affiliate: none" on the order page, and the vendor gets 92.5% of the retail price, less $1), or he can be his own affiliate, and send the traffic to his own affiliate hoplink and earn both the vendor portion of the sale and the affiliate commission divided between two separate accounts. (Or he can have additional affiliate accounts, all in his own name but making separate sales, which some vendors do as a way of boosting the product's gravity figure, because gravity measures the number of affiliate accounts each making 1+ sales, rather than the number of sales).

Until recently, Clickbank used to disclose in the marketplace statistics a figure called "%rfd" which purported to measure the proportion of sales accredited to affiliates. It was largely nonsense, of course, because the figure could be 100% and many of the sales might still have been made by the vendor himself, using one or more affiliate account hoplinks.