In a medium sized pot over medium heat whisk the vegan cream, vanilla, and sugar together until well combined and the sugar has melted. Next bring the mixture to a boil and immediately add the agar agar. Whisk the mixture vigorously for two minutes then remove the pot from the heat. The agar agar only needs to boil just long enough for it to activate, about two minutes. The mixture may seem not very thick at this point but trust me a little agar agar goes a long way and the panna cotta will firm up nicely in the fridge!
Pour the mixture in 4-5 heatproof moulds or heatproof glasses. I used silicone muffin cups for mine that are rather small so I made 5. Put in the fridge to set for about 1.5 hrs - 2 hrs depending on the size of the moulds you used.
Rinse the rhubarb. Depending on the size of the pannacotta moulds you're using, cut the rhubarb in appropriate lengths.
Heat and stir the sugar and water in a saucepan over medium heat until the sugar has dissolved completely. Poach the rhubarb and vanilla with sugar and water until the rhubarb has softened and sweetened. Remove the rhubarb when it has softened, but not become too soft that it's almost mushy. Save the rhubarb syrup for another use later, use the syrup in drinks for example, or over ice cream. Let the rhubarb and syrup cool.
Remove the panna cotta from the moulds and decorate with rhubarb and lilac syrup! Enjoy!