Yes, "vostri" means "your (pl) plural masculine things" but the English sentence "I have all of your sandwiches" is ambiguous. It could mean "I have y'all's sandwiches" and it could also mean "I have every single last sandwich of yours." I think the latter is the more likely interpretation.
The correct translation is roll. Panino means little bread. Roll is the most correct translation and in most Italian dictionaries and language sites roll is the main translation for panino.
Panino is also used as short form for panino imbottito in Italy. The correct translation for panino imbottito would be sandwich.
All Italian sentences on DL I came across so far except one, should be translated with roll. Some already are corrected and now accept roll, others not yet.
The only exception of the sentences I mentioned, where I would consider sandwich to be the right translation and roll would be wrong, is a sentence that I remember as something like bread is becoming a panino. Here I would consider the use of roll wrong because technically an already baked bread can hardly become a roll (only a slice or a part of the bread) and you can safely assume that this sentence is about the short form of panino imbottito.
il tuo, i tuoi, la tua, le tue ==> all of these mean belong to singular you. The difference among them is whether you own one or plural things, and whether those things are grammatically masculine or feminine.
il vostro, i vostri, la vostra, le vostre ==> all of these mean belong to plural you (y'all). The difference among them is whether y'all own one or plural things, and whether those things are grammatically masculine or feminine.
It's always "io ho". Saying "io ha" is like saying "I has".
Regular verbs conjugate according to one of three patterns, depending on how the infinitive ends:
But avere is irregular: