Does a Hazmat Vest make your soldier immune to Chryssalid poison?
From the source files provided with the mod tools, Chryssalid attacks inflict a 'ParthenogenicPoison' effect, one feature of which is dealing poison damage over time.
Hazmat vests provide both immunity to poison damage and a separate explicit immunity to the ParthenogenicPoison, so they will protect you against Chryssalid poison*.
*Or, at least, they're intended to: I'm not 100% sure that there are no bugs that might affect this (though I have checked that the Hazmat-vests-don't-block-acid bug doesn't affect it).