Is Cheque bounce non bailable offence ?

No. A Magistrate can take cognizance of an offence of dishounour of Cheques under Section 138 of the NI Act only on a Complaint from the holder in due course (in this case the person in whose favour the cheque was drawn). Hence, there is no question of any police investigation. Further, the offence is non-cognizable so the police cannot arrest without a warrant.

Suppose, the vendor files a complaint before the jurisdictional magistrate, the only circumstance in which you could be arrested before a conviction (if at all), is if you do not appear before the Magistrate pursuant to a Summons and subsequently do not take bail or appear before the Magistate, after a Bailable Warrant has been issued. To put it simply, just ensure that you are present or represented on the date on which you might be required to appear as per the Summons.

Furthermore, I must also mention, since you said that the cheque was dishnoured in Mar '20, for a complaint of alleged offence of dishonour of cheque to be maintainable, the holder of the cheque is required to issue a notice to the drawer (i.e., to you) within thirty days from the date on which the bank informed him of the dishonour, giving the drawer (i.e., you) fifteen days to make payment. If payment is not made within the said fifteen days, a complaint is to be filed within one month (unless the person can show sufficient cause for the delay). Going by the Month of dishonour you have given, it seems like the time-period to take action under Section 138 of the NI Act has expired. Further, you have not adverted to whether a Notice was issued to you to make payment within fifteen days.