I would ensure that backup vendors have been selected so that there is no impact to the project timeline as another vendor that has been pre-approved can be used instead. Oftentimes companies can have 2-3 backup vendors for such situations or even if the supplier lead times are not compatible with the project timelines. It's important to be proactive for such situations as it is not uncommon for this to occur. 

If the situation called for time pressing acquisition of product, I would immediately begin to contact other vendors. In most cases, I would assume there was a preliminary list of vendors being considered by the company before ultimately deciding on Vendor A. This means calling the next available manufacturer and finding out the possibility to resume production of the desired item. If the vendor never replies or even replies a week later, I would begin looking for a new permanent supplier because unexpected variables involved in the supply chain can be risky to any business.

I would also do the same as my fellow classmates if I was in a position where the vendor was no longer responding to emails. I would first try and contact someone else from the company to ensure that not just the one point of contact was not answering. Simultaneously, I would reach out to at least two other backup vendors that were already chosen in case a situation like this occurred. If the supplies needed are on the critical path and needs to be received before the project moves on, I would go with the backup vendor if they were more responsive. This is because I would not want to waste time if the vendor was not explaining why they were not answering. I would find this unprofessional and would want a more reliable company to get the job done. If this was not on the critical path and had the best product, I would consider giving it a little bit of extra time to get the supplies that I had originally wanted. This would still be unprofessional to me, however, and after a few days would still highly consider switching vendors. Is there ever a situation where you would give the vendor the benefit of the doubt?

Contrary to some of my peers I wouldn't start reaching out to other suppliers immediately but I would already have other vendors lined up and backup/substitute plans for emergency situations such as this. With any type of business, you have to rely on a structured system to keep the business running and not individuals or vendors. You try as best as you can to not have the customer detect any hiccups because these are all metrics that affect your reputation and ultimately your success. Depending on what the product is too I would always try to have extra for situations like these. If it's a time-sensitive situation where a vendor couldn't provide in time I would reach out to sister companies for assistance to see what they might need in return, or in extremely desperate situations even competitors. It would obviously come at a cost but desperate times require desperate measures and the saying goes "Keep your friends close but your enemies closer."  

One of the main purposes of project planning is to detect possible problems and account for them in the schedule. This must also include accounting for issues with suppliers. There can be many different issues that can arise from working with a supplier such as faulty product, late delivery, or price increases. In any case, when a supplier is initially chosen they tend to be one of several options. Therefore, if one supplier stops responding to you, you should already have multiple alternative suppliers. Simply pick the next best option available. The priority should be ensuring that the project stays on schedule regardless of any issues with suppliers. 

I would do two things in this situation. The first would be to contact other vendors that make the same or a similar product that we need. This can even include retailers that may have the product that we need but are not the original manufacturer. As for similar products this will require some work to ensure that these products are substantially similar enough to be substituted and may lead to changes to the product if it is different enough. The second would be to reach out through different forms of communication such as in person or over the phone to try and get a response back. Obviously this may be seen as annoying but when a project is on the line the fear of being annoying should go out the window. You can even try to reach out to specific people in the company through email if the main contact is not answering.