It’s difficult to find vegan substitutes for uncommon ingredients like shrimp paste.
That leaves you with using alternative ingredients that are “close enough” to the taste and texture of it.
There are 2 vegan pastes that have a similar flavor profile and texture. They won’t be perfect, but should do in a pinch (especially since shrimp paste isn’t usually too crucial of an ingredient).
1. Miso Paste
Miso paste is made from fermented soybeans.
You can get it in a few different colors, though white and red miso are most common.
White miso is a lot milder and likely the better of the two to use instead of shrimp paste.
Red miso is much stronger, so if that’s your only option, you may want to use a bit less.
Marmite is a vegan condiment that’s made mainly from yeast.
It’s not always the easiest to find, but it’s fairly pungent (and at least somewhat healthy), and should be a reasonable substitute.
I would use either of these alternatives in a 1:1 ratio with shrimp paste in a recipe, but you can always start on the light side if you think they might be too strong.