I'll try to explain the steps to do this below. In this example I will describe how to add a custom field for a fine art website, where the custom field title is "Dimensions" (english) and then translate it into "Abmessungen" (german).
1. When setting up the custom field, instead of entering "Dimensions" as a title, enter "COM_VIRTUEMART_ART_DIMENSIONS" (without the double quotes).
2. If you have i.e. English and German languages installed in Joomla 1.7.x, locate your frontend language files - 'Overrides' folder. There should be an 'en-GB.override.ini' file already that contains only the usual header information for language files but doesn't contain language keys/strings yet (unless you added some overrides already). You can create a copy of this file, rename it to 'de-DE.override.ini' and place it into this folder too. (Note: You have to use an editor that is configured to save the files as "UTF-8 without BOM, i.e. the Notepad++ editor)
3. In your en-GB.override.ini enter the line (including the double quotes):
COM_VIRTUEMART_ART_DIMENSIONS="Dimensions"
4. Save the file encoded as UTF-8 without BOM
5. In your de-DE.override.ini enter the line (including the double quotes):
COM_VIRTUEMART_ART_DIMENSIONS="Abmessungen"
Note: The left part in capital letters is the Key, the right part after the equal sign is the String.6. Save the file encoded as UTF-8 without BOM
You are done! When you switch now from the english to the german product description, you should now see your custom field translation.

Screenshots of the result below (after adding some more custom fields)...
Update:Since version 2.0.6 the values are also translatable
So it is possible to have i.e.:
Color: red (english)
Farbe: rot (german)
You must add your values into an INI file
But some strings are hard to convert, you can for example use:
SIZE30_VALUE="30 CM"
SIZE70_VALUE="70 CM"
...and in your customfields you must then use
SIZE30_VALUE
SIZE70_VALUE
Update:For Virtuemart 3 you can also use this available plugin to get translatable custom fields without language keys:
http://extensions.virtuemart.net/products/multilingual-translatable-custom-fields-detail[attachment cleanup by admin]