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'.
The word strength has only one vowel.The adopted word from German, schnapps, also has only one vowel.
Armchair is an 8 letter word. The 7th letter is i.
housefly