I can’t speak for what is “allowed” or “not allowed” for Vaishnavs, because that belongs to another religion’s rules and traditions.

But from a Jain point of view (especially for those trying to follow a stricter path):

• Vinegar (including apple cider vinegar) is generally discouraged or avoided because:

- It is a fermented product – fermentation involves growth of many minute living organisms (nigod, bacteria, etc.). - Use of such products can increase himsā (violence) toward subtle life forms.

• Many serious Jains will avoid:

- Vinegar - Alcohol - Yeast-based highly fermented foods (beyond basic curd etc., depending on their level of observance).

So:

• For Vaishnav rules: ask a learned Vaishnav or their scripture-based authority.
• For a Jain or a person following Jain-style ahimsā-focused diet: it is safer and purer to avoid apple cider vinegar and other vinegars, especially if you are trying to live a more sāttvik and ahimsak life.