1. Missive 2. Envelope (but it may contain more than one)
envelope
An 8 letter word, with just one letter in it, is ENVELOPE. 8 letters in the word, 1 letter inside the envelope.
The only 8-letter word with 5 vowels in a row is queueing.
withdraw
The only word that matches is "BELIQUOR"
There is no 8-letter word that can be made, there are these 7 and 6-letter words though:senegasagenesagonesgenoasgnosesgossanseasonsegnossenega
strength
sentient
The only eight letter word that can be made from those letters is 'starting'.
Armchair is an 8 letter word. The 7th letter is i.
housefly
The word strength has only one vowel.The adopted word from German, schnapps, also has only one vowel.