(Warning:very limited expertise.)

Nothing obviously wrong leaps out from what you've posted.

But that isn't going to be any quicker to use than the equivalent Perl code:

    vec( $vector, $bit, 1 ) ||= 1;
[download]

A quick test shows that it is considerably slower:

use strict;
use warnings;
use Benchmark qw[ cmpthese ];
use Inline C => 'DATA';

my( $vec1, $vec2 ) = ( ( chr(0) x 125000 ) x 2 );
cmpthese -3, {
    inline => sub { mytest( $vec1, $_ )       for 0 .. 1e6-1; },
    vec    => sub { vec( $vec2, $_, 1 ) ||= 1 for 0 .. 1e6-1; },
};

warn "Different results" unless $vec1 eq $vec2;

__DATA__
__C__

int mytest(SV* sv_vec, unsigned int bit) {

    STRLEN vecbytes; // Length of vector in bytes
    unsigned char *myvec = (unsigned char *) SvPV(sv_vec, vecbytes);

    if (bit/8 >= vecbytes) return 0; // Check in range

    if (myvec[bit/8] & 1U<<(bit%8)) return 1; // Test if a bit is set

    myvec[bit/8] |= 1U<<(bit%8); // Set bit (CHANGES $vector)

    return 1;
}
[download]

Results:

C:\test>903727.pl
         Rate inline    vec
inline 3.11/s     --   -60%
vec    7.77/s   150%     --

C:\test>903727.pl
         Rate inline    vec
inline 3.10/s     --   -60%
vec    7.77/s   151%     --
[download]

You mention in a later post that "The real case applies some fairly complex logic to multiple large bit vectors (each 1m+ bits) which runs a lot faster in C.". That raises a couple of questions:

• If the real code is so complex, why are you asking us to make judgements based on such a trivial example that can never meet its stated goal of greater efficiency?
• The way you've asked the question suggests that you are unsure about the parameter handling rather than the actual internal logic. Where exactly do your doubts lie?

Update: There is also the question of why you are setting the bit conditionally? That is, your calling code will never be able to tell the difference between the situation where the bit was previously unset; and when it was already set.

So why bother testing if it is set and not just set it?