I'm sure many will find this guide useful. I spent day trying to figure this out until I found this guide http://forum.virtuemart.net/index.php?topic=127500.75 and added to it. Hope it will save you a lot of time.
How to create multiple variations of a product, where the choices are dependent on previous selection.
Like this example in Virtuemart demo (which does not work in the backend so it's not too helpful) http://demo.virtuemart.net/product-variants/multi-variant-detail
E.g. We have t-shirts in Sizes (S,M,L,XL) and Color (Red, Blue, Green, White). Size S is available in Color Red, Blue. Size M is available in Red, Blue, Green, White. Size L is only Color Red, Green.
We also want the product SKU#, prices etc. to change for each selection. So, we have to create Dependable Variants. Here is how:
1. Virtuemart> Configuration> Custom Field> Add a custom field that is type multi-variant. I named mine the 2 variants I wanted like "size - color" because this title shows up above the input fields. You will only use one multi-variant custom-field in a product. It is kind of a "mega" field with the variant fields defined within it - called ramifications. Here is how to set it up. Don’t forget to add Layout Position addtocart so it shows up on the front end below the product title.
After you save, another box will appear on the bottom but I didn’t bother with it.
2. Create a new product, fill out all the info (product name, sku & price, dimension, images etc.) Go to that product’s Custom field tab. Add the Custom field that you created for multi-variant, so in our case Size and Color.
3. A big box will open up. At the first dropdown, where it says product name, drop down and choose Custom Label. In the text area next to it, enter the label, e.g. size. (if you are using built in attributes such as weight, you pick that label from the dropdown box)
4. Enter the choices for the variant, one per line in the larger textbox to the right of the label text box. So first for sizes - enter the S,M,L,XL one per line.
5. SAVE - This seems to be important at this step and sets things up to add the next variant.
6. A bit lower on the screen you should see "New Ramification." Click the little green round button just left of the text "New." Don't try to enter anything into the dropdown or the text box for the custom label until you have clicked the green button and it comes back with empty text boxes for the custom label and the choices for the second variant, in our case Color
7. Now you should have a new dropdown with product name (select Custom Label,) a text box for the custom label and a large textbox for the variants, in our case colors. Again, type the colors – Red, Blue, Green, White. If you have more variants e.g. Material you can repeat steps 4-5 for the next variant.
8. SAVE
At this point you should have 2 variants, along with their choices, and a "New Ramification" box. Under the New Ramification should be a Add a Child Product button. If you had other variants, you could add them now by clicking the "New" button under the New Ramification. You could also add them later.
9. Add the child products. Click the Add a Child Product button as many times as you have child products. Or just start with 2 to test. You can add more later. The first time you click it, it will add a line for the parent and one child. Then each additional time for another child.
10. You should see a line for each child product that has a place for info like sku, price and also a drop down for each variant you added. The drop down will have the choices you entered in step 4.
11. Fill in the SKU & price if you want and select the variant combinations for each child.
12. SAVE. Note - when you go back to the product list, the child products will show "no price set." This is a small bug, if you open the child you will see the price carried over from the entries you just did.
13. Customize child products. You can then open the main product, open each child product and customize it to fit the variations it represents. E.g. if it’s a variation of a Blue color you can change the image.
Andrea