The W-9 that you would send out to your vendors typically would not have any information on it, and we see no problem sending it to them via e-mail. It would then be up to the vendors to return it however they see fit. Where there might be concern in sending it via email would be security of social security numbers for sole proprietors or single-owner LLCs, etc. -- but if they send it with some kind of security, such as a password-protected file, it should be fine.
The other consideration would be their signature. A signature actually is not required on a W-9 unless it is in response to a B-notice, in which case a signature is required. We will have to look into whether there is an electronic signature that is acceptable to the IRS in the latter case.
But as for W-9s, the IRS in fact does not *require* that you obtain W-9s from your vendors, unless you have received a CP2100 and sent a B-notice. However, as you know, you want to have a W-9 on file so that you have record of obtaining a tax ID from the vendor, along with their entity status, and it is best practice to get a W-9.
In case of a CP2100/B-notice, having a W-9 is a mitigating factor with regard to any penalty. You need to be able to demonstrate that you have exercised appropriate diligence in complying with 1099 filing requirements, including collecting a valid Tax ID from vendors, which the W-9 provides.
But we are unaware of any IRS restriction on sending a W-9 form via email.