By RAW, you can lie non vocally in a Zone of Truth.
The Zone guards against deception. PHB page 178 states
Your Charisma (Deception) check determines whether you can convincingly hide the truth, either verbally or through your actions.
In the spell description of Zone of Truth, they are not referring to Deception the skill, but the definition of the skill lines up fairly well with the english language definition, which states that deception is the act of decieving someone, and decieve is defined as making someone believe something that is not true, with no mention of having to speak.
This means deception does not have to be vocal, or even verbal. However, there is some ambiguity.
You cannot speak a deliberate lie. Speaking clearly means talking, making a sound with your voice in a certain language. Body language, sign language, and pantomime are not speaking.
By RAI, you cannot lie non vocally in a Zone of Truth.
They say speak a deliberate lie, yes, and RAW must interpret every word of the spell description. However, it seems obvious to me that they intended no deception to be possible, as shown in the earlier line.