I think you're right that we may be talking about two different things. When I was looking up specs, I saw several reviews that stated it had an expresscard 54 slot.
https://www.cnet.com/products/dell-vostro-1510-laptop-computer-intel-core-2-duo-t5670-250gb-3gb-series/specs/
https://www.pcworld.idg.com.au/review/dell/vostro_1510/222023/
But some of the reviews indicate it came with an Nvidia 8400m or 8600m, while others did not. That's not unusual, if it was an optional feature. But every mention of a GPU indicated it was soldered to the board, usually in reference to the GPU dying and needing a full mobo replacement rather than just the GPU. But then I saw this:
https://www.amazon.com/Genuine-Dell-GeForce-Graphics-Inspiron/dp/B002N7IX5M
Which does in fact appear to be a Dell OEM internal GPU that fits the express card slot. As opposed to a product like this:
http://www.hwtools.net/Adapter/PE4C%20V3.0.html
But to get that to work, you're looking at >$100 just for the adapter, then however much the GPU costs, and as I mentioned, the high likelihood that an old Dell laptop's BIOS won't recognize the card. So I'm with you on it not being worth the cost. I just can't say for certain it would even work in the first place. It was only seeing that card on Amazon that made me think upgrading the GPU on that laptop might actually be possible, albeit ridiculously priced.
When people talk about egpu's now, it's usually because they heard about Thunderbolt equipped laptops that support the feature.