With one "s", as in "vision", the "si" forms the voiced sound sometimes shown as "zh" or as "Z".
With the double "ss", as in "impression", the "ssi" forms the unvoiced sound usually shown as "sh" or as "S".
"vision" = /v
iZ^n/ "impression" = /impr
eS^n/ /^/
represents any of several indeterminate central vowel sounds which
typically occur in unstressed syllables in English. In these, use
the /i/ or /u/ (as in "bit" or "but") or anything in between for the
/^/.
The /S/ and /Z/ are produced the same way except for the voicing of
/Z/. The tongue is held more forward and curled than for /s/ or
/z/ and the lips are more rounded.
These are difficult to describe - hope this helped!
Jim