Portamento is a continuous movement from one pitch to another, with no tones in between. It is a style that you can hear in Tommy Dorsey’s trombone playing. Even though he was playing across “partials”, it was as if they didn’t exist. Although he was great at this, occasionally, you’ll hear a little “flaw”, as it were, with a tone making its appearance between two notes. It is very difficult to play this “super-legato” style on trombone across partials and takes a lot of practice. I was taught this on trombone by a Las Vegas trombonist who had the fortune of having studied with Tommy Dorsey’s teacher. In singing, portamento is easier for me, than on trombone.
Tremolo is a quick “reiteration” on the same pitch and the pitch does not vary in frequency, ideally. I’ve heard a very few singers emulate this by using abdominal muscles quickly contracting and relaxing. It’s not the same as vibrato and can sound “nervous”.
Vibrato is the pitch varying downward and up, sort of like a sine wave. “Straight tones” have no vibrato. The speed and depth of the frequency oscillation can be variable. Some singing teachers or vocal coaches have stated that 6 to 7 undulations per second is ideal. However, you may notice that vibrato was faster in popular singing in the 1940s than it is today, when you find examples on YouTube of 40s singers. A very slow and wide vibrato sounds comical to some people, and it often is heard in elderly singers as a result of muscle atrophy and/or lack of control or concept.
Since we are talking “tones”, this is applicable to both voices and musical instruments.