Hi, and welcome to the Apple (user-to-user) Community.
Re: "ordered five months ago but were never received".
Wondering if this purchase was from a reputable, known company, or not.
( It seems they accepted Apple Card ok.)
Re: "I’ve attempted to contact the vendor’s customer service multiple times, but I never get through".
Seems the vendor is the one who would have your money. If you can not phone them,
maybe see if you can get to "on-line chat" with their staff via their website, or find an address to eMail them. Look for a "support" webpage, or a link to "contact" details.
Re: "The form Apple provides to dispute charges doesn’t have “never received” as an option."
Guess "never received" may be more to do with the vendor. Maybe you could just choose the nearest option, or see if you can get to chat with an Apple Card specialist from sending a message, as the article below says.
How to: Dispute a charge to your Apple Card - Apple Support
Re: What can I do to get this charge reversed?
If nothing above has helped:
Try asking Apple Card Support what else you can do.
(There is a "Get Support" link (online chat, phone, eMail etc)
at the bottom of their website (link below)
Apple Card - Official Apple Support
All the best :-)