Personally I think if you make a change in your life related to this you should do one thing at a time so you know what works.
If I am not mistaken, which I am often, then you are consuming some food items that contain caffeine. I notice that the more caffiene that I consume the more tierd I become later on. I think the simplest idea would be to take walk to get the blood flowing, but do it outside to get more oxygen in your system. Maybe even just relaxing in the outdoors will help as well. I have the same problem as you during the week because I am sitting all the time during the day. The week ends are different because my routine is varied.
If you are able to do some weights at your facility then you may want to try that too. Even though you are on your feet all day your body is used to it so it may be similar to sitting behind a desk all day.
I can't comment much on your diet because I do not know much about a vegetarian lifestyle.
Another interesting homework assignment for you is to research hormone pathways and the way they relate to circadian rhythms so you can understand what forces are at play in your body and how they are making you crash at 1430. Then try to play around with how your body hormones with activities and the things you eat. Play investigator to see what works best for you.