I can confirm the problem for 4060380287, both with 5.14.2 and 5.15.8 (64-bit)

$ /usr/local/perl/5.14.2/bin/perl -we '$v=""; vec($v,4060380287,1)=1'
Negative offset to vec in lvalue context at -e line 1.
$ /usr/local/perl/5.15.8/bin/perl -we '$v=""; vec($v,4060380287,1)=1'
Negative offset to vec in lvalue context at -e line 1.
[download]

but not for 2**32 (and above — in which case it wraps around¹).  Actually, it seems 2**31 .. 2**32-1 is buggy, though I haven't tested each and every one individually.

For reference, my relevant config settings:

...
  Compiler:
    cc='gcc ', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasi
+ng -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -
+D_FILE_OFFSET_BITS=64',
    optimize='-O2',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -f
+stack-protector -I/usr/local/include'
    ccversion='', gccversion='4.3.2 [gcc-4_3-branch revision 141291]',
+ gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+6
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t',
+ lseeksize=8
    alignbytes=8, prototype=define
...
Characteristics of this binary (from libperl): 
  Compile-time options: HAS_TIMES MULTIPLICITY PERLIO_LAYERS
                        PERL_DONT_CREATE_GVSV PERL_IMPLICIT_CONTEXT
                        PERL_MALLOC_WRAP PERL_PRESERVE_IVUV PERL_USE_D
+EVEL
                        USE_64_BIT_ALL USE_64_BIT_INT USE_ITHREADS
                        USE_LARGE_FILES USE_LOCALE USE_LOCALE_COLLATE
                        USE_LOCALE_CTYPE USE_LOCALE_NUMERIC USE_PERLIO
                        USE_PERL_ATOF USE_REENTRANT_API
[download]

___

¹ as I can find no mention in vec that the offset is limited to 32-bit on 64-bit builds, this could be considered a bug, too, even though it doesn't croak in this case.

$ perl -wE '$v=""; vec($v,2**32+42,1)=1; say "wrapped" if vec($v,42,1)
+'
wrapped
[download]