Tip for Talkloiding: Thinking noises. We all make them, but some people make them differently, according to the individual as well as the accent/dialect. So...
English:
-Several Vocaloids are American (Dex/Daina, Ruby, Cyber Diva/Songman, etc.), as well as most English SynthVs (excluding Maki, NineZero and XLS cases). The standard thinking noise in American English is "um"/"uh". [V m]/ [V]
-Some Vocaloids are from England (Leon, Sonika, Oliver). An accent like Oliver's would probably use "er" [@], but I think Sonika's accent is more northern and she might use "eh" [e] similar to Avanna. (Also, I don't recommend trying to use [@] with Sonika, her [@] phoneme is very finnicky)
-Avanna has an Irish accent, so her thinking noises are most likely "em" and "eh". [e m]/[e]
-Sweet Ann and Ninezero have Australian accents, and I think Australians mainly use "uh" like Americans do.
Other Languages:
-In Japanese, "ee to" and "ano" (first and last syllable stretched repsectively) are standard. Japanese-accented vocaloids could technically say this in English, though I tend to have it that they learned to use English thinking sounds (usually "um"/"uh" because most JP-EN Vocaloids lean American).
-For Mandarin I'm not sure. After a bit of searching I heard they often say "neige" which means "that" (and I guess would be the equivalent to the English use of "like"), but for a thinking sound I've heard they say "mmm".
-In Spanish I think it's "e", "em" or "este" according to Wikipedia; I think I've also heard a "la la la" type of sound (I live in California so I've heard native Spanish speakers; it's just been a while), which I think is like an English speaker saying "I went to the, the, the theater last night" when trying to think of the word.
-Apparently in Korean it's "eung", "eo" (similar to "uh" i guess), "geu", and "eum".
I think it's a universal thing to extend the first letter of the next word as you're thinking of it on occasion: "So is that the sss...second one?"