Best way to find out is to ask them directly. Not sure if a rep will stumble into this thread and respond but for the most part you're just going to hear from us customers. Maybe someone has tried to do it or was successful and they will answer.
I just did it and it worked fine. Here is what you need to do.
Buy a VERIZON iPhone 6 from an Apple store. Tell them you want to buy it outright and they won't ask questions or force a contract. Do NOT buy the unlocked iPhone, as that will only work with ATT/TMobile I believe. Also the unlocked iPhone does not come with a verizon SIM card, which you might want. The Verizon phone DOES have the sim card.
Activate the phone online using My Verizon. Enter the IMEI and the ICCID (SIM card number).
Turn off the phone, wait about 5 minutes, and then turn it back on. It will activate.
If you try to buy a full priced iPhone 6 from a Verizon store for a prepaid account, they won't sell it to you. They will say it won't work. They have a policy to only sell the iPhone 6 for a postpaid account. I can tell you that it does work though, because I literally just did it.
If you look at "compatibility" for the Verizon 4G LTE Prepaid SIM Card online, both the iPhone 6 and 6 Plus are listed as compatible.
If you are switching from a 3G device to LTE, YOUR CURRENT MONTH WILL BE CANCELLED AND YOU WILL BE CHARGED FOR A NEW MONTH. The Prepaid 3G service is NOT the same as the Prepaid 4G service. It says this nowhere on the website, so if you just signed up for a month they might help you if you call and explain that the website does not warn users of this. (The website doesn't even distinguish between 3G and 4G smartphone plans at all.) If you don't want to be charged for the new month, switch it at the end of your month.
If you have trouble activating it, try activating an officially supported 4G phone and moving the SIM card to the iPhone.
Apple has a 14 day no-questions-asked return policy. If it doesn't work within 14 days, return it and try again.