Most of what I have been taught in choir training has already been said. A few elaborations. though.

Let me underscore WYSIWIG's point that many vowel sounds in English are diphthongs, and there's a way to sustain them without sounding screechy.
Long "i" is actually "ah-- ee," and long "a" is actually " eh-ee," and "how" is actually "ha-oh," for example. It generally sounds better, even in folk music,
I think, to sing "aaaaaa-ee," "ehhhhhhh-ee," and "haaaaa-oh" than "a-eeeeee," "eh-eeeeeeee," and "ha-ohhhhhhh."
We are often encouraged in choir to sing "eh" pretty much all the time instead of "eh-ee" -- e.g., "Oh, what a beautiful dehhhhhhhh!" We may put a
teeny weeny "ee" at the very end, but you'd be surprised how natural the diphthong vowel sounds sound when sung with only a bare nod to the second
part of the diphthong.

FWIW, it seems English is one of the few languages that uses the long "a" sound and American English is one of the few places you will hear the kind of
"r" we yanks use.

In Spanish singing, sometimes the rolled "r" can be sustained to great effect, and I think "mmmmmmmm" and "nnnnnnn" can sometimes be sung to
great effect in some musical styles. But the "American 'r'" can be really grating when sustained. Occasionally even the "ssss" if effective. ("I got [clang,
clang] ssssssteam heat....".)

Putting the final consonant of a word at the beginning of the next is a good technique in choral singing, to make sure you don't hear something like
"this-s-s-s land-d-d-d iz-z-z your land-d-d-d-" from your trio. It's less important when singing solo, but even there it can help keep the "s" sound in
check. You've gotta be careful though (as pointed out in another thread) when singing phrases like "the Lord rules" or "lead us not into temptation."
*G*

Again, I'd say try taping yourself singing the song or phrase both ways and see which sounds better for the particular song or style.