I traveled to London recently. My friend told me that the dressing I used on my salad comprised many ingredients including white wine vinegar. Was it unlawful to eat it?

According to the consensus of Muslims, the salad dressing you consumed is lawful. This is because vinegar is generally lawful and is one of the wholesome foods based on the general implications of the Prophet’s words: "Vinegar is the best condiment."
Based on this, if an intoxicant or alcohol transforms into vinegar, whether spontaneously or through the manufacturing process, it is lawful [to consume] according to the consensus of Muslims.
And God the Almighty knows best.