Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
Spread the almonds and hazelnuts into a single layer on the baking sheet and roast the nuts for 17-18 minutes. Remove from the oven and let cool for at least 10 minutes.
If using hazelnuts that are not blanched, transfer them to a clean towel and rub them to remove most of the skins.
Add the white sugar and water to a large skillet or saucepan (preferably with a light bottom to see the color of the caramel). Heat over medium heat to melt the sugar. Once it starts to bubble, keep heating until it turns golden brown. You can use a wooden spatula to stir from time to time until the caramel has a light brown color.
Add the roasted nuts and the vanilla bean to the caramel and stir to coat. Heat for another minute, stirring frequently to coat the nuts with the caramel.
Transfer to a baking sheet lined with parchment paper or a slipat, and use a spatula to spread it into an even layer. If there is still melted caramel in the skillet, transfer it over the nuts as well.
Let cool completely, about 1 hour. Next, break the praline into small chunks to fit it into a food processor or blender. Be careful not to cut yourself with the hard caramel.
Process for 3-5 minutes, or until the praline nuts turn into a smooth and almost liquid paste. Scrape down the sides if needed. You can stop once it has turned liquid but still has a crunchy texture or process it longer for a very smooth paste.
Transfer to a clean jar and store at room temperature. Praline will keep for months. If it separates, give it a good stir.