Should I trust packed sandwiches in cafes (containing pesto) stating they are suitable for vegetarians?

I would trust it to be vegetarian but not vegan.

Something with parmesan (or other cheese) would be generally considered vegetarian.

I would not trust anything packaged to be vegan unless I knew the maker well and it was labeled as vegan.

Many vegetarians do not eat cheeses that contain rennet. Most Parmesan cheese contains rennet. I would not trust that basil pesto is suitable for vegetarians, unless it specifies that the pesto does not contain animal rennet.

There is microbial rennet that is suitable for vegetarians, and it is always listed as microbial rennet.

Vegans do not eat any cheese that is made with animal rennet or animal milk products.

Has a sandwich ever lied to you? Sure, I’d trust a sandwich.

Vegetarian yes, vegan no. Just as @zenvelo stated.

@Kardamom Vegans don’t eat any cheese period. Maybe you meant to write vegetarian in your last sentence.

Vegan cheese is a thing, @JLeslie

@NerdyKeith As stated above, “suitable for vegetarians” is not the same as “vegan”.

@Muad_Dib Vegan cheese is a thing

Cheese, by definition, is a diary product, made from milk, and therefore can not be considered vegan.

What is sold as “vegan cheese” is made from vegan products, often with legume or tree nuts, then flavored to closely resemble the dairy version. Some of these products are actually quite flavorful, and do come close in flavor and texture to their dairy equivalent.

However, if someone is eating vegan because of dairy sensitivities, they need to be aware of any other sensitivities, such as nuts, gluten, etc.

Finding rennet-free things in an ordinary deli-type circumstance can be a challenge. Very often non-vegetarians are not even aware of the rennet problem.
I would not trust the sandwich to have rennetless Parmesan unless it was specified as such, before you ask.
At least there are rennetless alternatives available nowadays, back in the day there weren’t.
Worcestershire sauce was another problem back in the day.

@zenvelo, the rennet issue is a very real concern to many vegetarians who are not vegan.

Quickie search turned up a partial list of cheese brands that use microbial rennet here.
You’re Irish, right? There may be a comparable list for where you are, and if the café employees can identify by brand, you may be able to sort it out.
Good luck!

@JLeslie, no, I meant vegans.

There are “cheeses” that are made with nuts, and soybeans, and flax, and hemp etc. Vegans can eat these types of cheese.

Some vegetarians (although I don’t really consider them to be vegetarians) will eat animal rennet types of dairy product cheese, but most will not. The reason they won’t eat it is because to make it involves the death of the animal. Vegans will not eat dairy cheeses with or without rennet.

Vegan Cheese

@Kardamom I consider those fake cheeses. My sister is vegan so I know what you are talking about now, I just don’t think of it that way in my head.

I’m with you in the rennet. I understand why some vegetarians are concerned. I hadn’t thought that through. It’s a touchy thing with kosher diets also. Most rabbis are ok with it if the animal was killed kosher style. It’s an odd exception in my mind. I think most kosher cheese is not animal rennet.