Difference between Vietnamese and Moroccan preserved lemons?

There's probably not much difference.

There is a fair amount of variation in preparation methods for Vietnamese preserved limes and lemons (chanh muối). The Garden Betty recipe you linked is probably the most common technique - soaking nearly-quartered lemons in heavily salted water for a few weeks - but that may not be the same method used for the $3 jar you found. Sometimes sugar is used in addition to the salt. Like your Moroccon recipe, sometimes no water is added and the salt instead draws juice out of the fruit to supply the preserving liquid. Boiling the lemons before jarring them is very common. I've also heard of other pre-jarring routines like scrubbing them with salt, sugar, and even tea, or letting them partially dry in the sun first. They may also be jarred in different forms, such as thin slices or left whole.

There is also the obvious case of what actual fruit to use. Because of the nature of citrus fruits, there isn't really a clear divide between limes and lemons in the first place, and there are quite a lot of varieties and hybrids that make classification difficult. I live in the US and use Meyer lemons and Key or Persian limes (which themselves are actually lime-lemon hybrids). The Moroccan recipe calls for doqq and boussera, but suggests Meyer lemons if the traditional varieties can't be found. In Vietnam the fruits are called chanh tây and chanh ta, but I'm not actually aware of a definitive conclusion on which one is technically a lime and which one is a lemon.

Overall, I'm not familiar with different spins on the Moroccan approach, but I assume there's a similar range of variance. That said, it seems like it's basically the same process. Besides the choice of fruit - which, outside of Morocco or Vietnam, is probably going to be the same anyway - I think it's safe to say that any differences between the two cultures are indistinguishable from differences between variants within the same culture.

Of course, the easiest thing to do is just spend the $3 and find out how they taste!