Hello Ali,
I have downloaded the Vivaldi font and tested this issue. It seems it is working fine. I have created a sample and attached the sample with this post. You can get the sample from the following link -
http://websamples.syncfusion.com/samples/Diagram.Windows/F61507/main.htm Sample Details :
1. Creates custom symbol that has text node as a child node. This is demonstrated in sample's MySymbol.cs class.
2. The text node's text font family uses "vivaldi" type font.
Also please make sure that you have the TextNode.FontStyle.Italic is set to true.
Please look into the sample and let me know if you have any more queries.
Thanks,
Meera.