I just uploaded some ogg files onto the wikia of Prima, Sonika, Big Al and Tonio last night. You can study the vocal results in a DAW if it displays the singing pitch, it actually brings out into the open any word that came out differently (its slightly louder) to the others. In fact you can study the vocals and compare them to a real singer in FL studio if your prepared to spend the time and don't have the capablities to do things manaually without extra software involved.
Also, some notes from this;
*Off the bat with no vocal editing, you can hear on the wikia's ogg files who is singing naturally better then the rest. Prima is prob. the Vocaloid that off the bat with no editing the best vocaloid hands down. She just sounds the best overall, of course we know how much you need to do to make her come out of her style. She is also the most stable vocal
*Tonio's "r"'s just don't sound correct, I place him as the weakest in "r"'s of the four, he is also awefully quiet across most of the notes except every odd one.
*Off the bat with no vocal editing, Big Al REALLY croaks on short notes, he doesn't like half a note, he wants at least one whole note. However, off the bat I place him second since his vocal results often come out relatively close to Prima's when you study the file results in a DAW.
*In some cases you can literally SEE where the diphones are kicking in...No really... the DAW reveals all. In a natural singer the transition is smooth, note for note you can see where each section of a word is kicking in. In some cases its quite dramatic. For instance in Prima's file, the start of "sing" you can see in the DAW is quite gummy but the "ng" (end of "sing") is quite clear and lourder. So you would need to focus on improving "si".

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Edit:
You can actually use any software btw for this that displays the vocal outputs. If you've got the software you can also fix this problem anyway.
Also I added Prima's vocal results on the word "Sing".
As I said, you can see where the second half of the word is kicking in at the end, btw a natural singer wouldn't be so noticable like that for the curious. Prima was holding the "si" in the note to sing this because it was a long note, Vocaloid tend to hold the first part of the word when they sing long notes, they insert the rest in quickly at the end (sometimes its sloppy how they do this). You have to break them down further then a single note to make them hold anything else. Sing is almost one whole word...