C# Coding Standards – When Are Chained Assignments (a=b=c) Bad Form?

There's no truly compelling reason to force someone to change languages unless one has a feature that's going to be particularly useful or time-saving for your project(s). They'll both compile down to IL and perform equivalently (assuming that Option Strict is on in VB.NET...otherwise you can incur penalties for late binding). Everything else truly is preferential (not to dismiss that at all, but it's not an objective metric).

I would suggest looking at job listings in your area and see which language is more prevalent both in work supply (i.e. the labor pool for both languages). Seeing which one will provide you with a bigger or better labor pool is probably going to be your most compelling metric.

You have the benefit of viewing the code with fresh eyes. Take notes to document what you discover of bad practices. Then, as you're getting settled with the team, pull out your notes at an opportune moment, like when it's time for refactoring.