Was it considered a sin for me to consume the vegetarian cheese sandwich if I had concerns about potential cross-contamination with Haram food at the restaurant?
No, it is not a sin to consume vegetarian food for any lawful reason. Those who claim that non vegetarian food is totally bad and don't consume it all, are doing sin because they denying Quranic verses to eat meat and the practices of the Prophet (SAWA) and his Ahlul Bayt (AS) who did eat meat.
'Wassalam.