The prototype of the respective function (doop.c, line 741) suggests the offset is in fact 32-bit:

UV Perl_do_vecget(pTHX_ SV *sv, I32 offset, I32 size)
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at least if the type I32 always is 32-bit — which it seems to be...  as it turned out in a recent thread.

(As it's a signed int, the largest positive value is 2³¹-1, or 2,147,483,647.)