This is definitely the word to describe what Vivian does in the middle of the night when she is sleeping. I think she's got a little Swiss Miss Cocoa action going on. It might even be a slight yoodel, I'll get back on the verdict after a few more evenings.

Dictionary.com Unabridged

chor·tle   verb, -tled, -tling

–verb (used without object)
 1. to chuckle gleefully. 
–verb (used with object)
 2. to express with a gleeful chuckle: to chortle one's joy. 
–noun
 3. a gleeful chuckle. 

American Heritage Dictionary

chor·tle

n.  
 A snorting, joyful laugh or chuckle.
intr. & tr.v.   chor·tled, chor·tling, chor·tles
 To utter a chortle or express with a chortle.
 

[Origin: b. chuckle and snort; coined by Lewis Carroll in Through the Looking-Glass (1871)]