According to the Eureka Part diagram for your particular model vacuum, your vacuum uses Eureka Style U Belts. Eureka makes actually 2 different styles of U belts.
The first style is the standard U belt, Eureka part number 54312. This belt is probably the one that came standard on your model. You may purchase this belt from the link below;
The other type of Style U belt that will fit your model is the Eureka Extended Life Style U Belt, Part 61120B. The Extended U belt is more durable, and lasts longer then the Eureka Standard U belt. You may purchase the Extended U belt here;
Now, if you are quite sure how to change your belt, you may visit the link below to see the owners manual for your Eureka 2961BVZ that will give you step by step instructions on how to change the belt;
I really hope that this was helpful to you in finding and fixing the belt for your Eureka. If you did find this helpful, please be kind and rate my repair solution as helpful by clicking the thumbs up icon in the lower right hand corner below. I really do appreciate it, thank you.
Feel free to request a repair clarification if you have any questions about the procedure of installing the belt on your Eureka, I'll be happy to help.