Yeah I must admit I'm not familiar with the Sia project. I'm sure Sia has merits and quality LOE.
However Vcash is on a whole another level. Over the past year John introduced Chainblender (big deal) along with significant/major improvements on security, scalability, and performance. All of it, new code. This doesn't even include the amount of new code/work half a year prior which included the ZeroTime release and core updates (which is also huge deal). For an internet currency XVC is the best in it's class. Vcash has phenomenal technology under the hood compared to its predecessors before it.