Precisely.
If Nanoha can put two and two together that's great, but that's different from outright stating in a room full of potentially hostile
humans we're an unholy monster of legend that
eats humans. The CDC are jerks, they don't need to try to be reasonable when they've got an excuse to be ruthless and they're not going to pass up a chance to one up Nanoha's division especially when that's the whole reason they're here.
If we drop some hints that point to why we need blood and someone figures it out that's great. But outright stating it is going to result in heads rolling when shit hits the fan.
Plausible Deniability is absolutely crucial. For example:
- Let's say Flandre tells Nanoha we're a vampire. Whether or not she believes us or outs us irrelevant. When we start using our powers and eventually properly Feed the CDC can come out and say "This was all preventable. We're all such badasses we totally would have secured the vampire, but those losers knowingly withheld the truth about the monster from us until it was too late!" Nanoha and co. will be lucky to get a court-martial.
- Now what if Nanoha or someone else puts the pieces together on their own? Then when Flan starts being Flan Nanoha and peeps can fire right back that we looked and acted just like any other girl while in their care. The CDC can't throw blame at anyone when their own goons came sniffing around and were totally fooled too.