The sides meet in Austria on Wednesday, with Zenit's opponents yet to record a win in Group G.
A triumph for Zenit would take them into the knockout stage and, depending on the result between Atletico Madrid and Porto, a draw or a defeat may also be enough.
Despite Vienna's winless group campaign so far, Spalletti is anticipating a tough encounter after the sides' last meeting ended goalless.
"The game is very important for us. I am sure Austria will take the match very seriously and like all the teams, will want to win in front of their home crowd," he said.
"The match will not be easy, Austria will give their best, it is a Champions League match in the end. They will not make it easy for us."
Zenit have won only once in the group so far, beating Porto in October, drawing three of their five matches.
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