I’m not sure if the Vellum theme is compatible with the Polylang plugin. The only multilingual plugin that has been tested with Vellum is WPML.
With regard to your second question, if you can’t find an answer to that using the forum search please create a new topic for it.
Please try to remember to start a new topic for each separate issue that you have. We like to keep each topic in the Help Center focused on a single subject. We find that separating each question into a separate topic keeps the information much clearer and easier to follow. Also, it really helps other users to find information and it prevents threads from wandering down Tangent Lane. If you can’t find a solution using the forum search, please always create a new topic for each separate issue that you have. Thanks.