vwpartsvortex, vwdirectautoparts, and some others are separately run websites for some service center parts departments. VWpartsvortex is out of NJ, VWDirectAutoParts is somewhere in Texas I think. They are not tied to parts.vw.com which is the official VW parts site, although they sell the same things. Generally partsvortex/directautoparts/etc will have some sort of discount on everything.
I've placed plenty orders from both, but find they are both slow to respond to emails. They only (in my experience) source OEM parts so seem to be subject to the same overall supply constraints as any dealer service center you might go to. However, they often have parts that no longer show up parts.vw.com, even though you could go into a dealer service center and order them.
What I have noticed is that partsvortex/directautoparts/etc sometimes list parts that you can't actually order. It seems to be a mistake in what must be a (semi)automated inventory system. I made a thread a few weeks back asking about a suffix that showed up for an MIB 2.5 unit that was listed on partsvortex. Turns out that suffix is VW's internal code to track broken units that have been replaced. I didn't place an order but I assume it would have eventually been cancelled.
If you go to parts.vw.com you can put in your (or any) zip code then check the dealers that are close. This lets you "shop around" to see who is offering a discount, which you can compare to what partsvortex/directautoparts/etc are offering. I'm in SoCal and found that VenturaVW almost always offered the same discount as those sites but they are about 40 miles from me so I would order from them. That meant I could pay economy shipping but would get the part next day (if I didn't want to go pick it up).
What is frustrating and silly is that I would call my local dealer (5 minutes away) and they would have a part in stock but offer no discount. I would then call VenturaVW who would show no stock but would confirm the closest stock was at my local dealer. I would then order online from VenturaVW (at their discounted price) and internally VW would ship it from my local dealer to VenturaVW... who would then ship it to me
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