VIPs

The -s attaches to the letters themselves, so the initialism behaves like an ordinary countable noun: one VIP, a roomful of VIPs, with no need to pluralize a word buried inside the phrase. Most readers meet it as very important persons, but the same letters also abbreviate vasoactive intestinal peptide and ventilator-induced pneumonia in medical writing, and the plural serves those too. Context, not spelling, tells them apart.