@whale-av said:
and brown paper envelopes are a rarity nowadays.......
You are confusing "manila" and "vanilla." Manila is a fiber like hemp which is used to make various things and they all end up that sort of brown color. Vanilla in this use just means plain and unadorned, like plain old vanilla ice cream with no toppings. Fairly common usage, at least in the US.
Edit; That was somewhat pointless and pedantic. Seemed very relevant and on topic while I was waiting for my coffee to brew.