The target pronunciation is inferred: Oxford Learner's transcribes viciously as /ˈvɪʃəsli/, pointing to /ˈvɪʃəs/ for vicious, stress on the first syllable, in UK and US English.
Physical cues
Press your top teeth lightly on your lower lip and hum with voice for the v. Keep it short, then open straight into the quick vowel /ɪ/, like the i in bit.
Push the sh sound /ʃ/ next. Raise the front of your tongue toward the roof of your mouth, close to but not touching it, and let air pass with a soft hiss, as in ship.
End weak and relaxed. Say /əs/ with a tiny restful vowel, like the a in about, then a light s. Keep the second syllable short, since the stress belongs on the first.
In a phrase
Say it in a set phrase: a vicious cycle. Hit the first beat hard on VI, then let /ʃəs/ fade quietly behind it.
Try a longer line: That dog has a vicious bite. Keep the first syllable strong and the ending quick, landing as /ˈvɪʃəs/ in both US and UK English.
Compare endings on the word itself: say vicious, then say only the tail, /ʃəs/. The first syllable carries the beat while the second stays soft and short.
Contrast the opening sound: vicious starts with a voiced v, fish starts with a quiet f. Whisper fish, add voice for the v, then say vicious.