So, if you want to achieve a fp with a crescendo effect in Sibelius with Vienna you can follow these steps:
On the second row of your sfz patch, load a sustain patch in the slot under the sfz. I adjust the volume of the sustain to be a little lower than the sfz. Just a db or two. This is up to taste. I also make the velocity curve more dynamic by setting the lowest volume to around -48db.
Alternatively: Load the sustain patch on the second row of your sfz patch, if you want to use the regular sfz in your score as well. Then just put the text ~C1,127 over the score where the fp is to begin. The Vienna sound set seems to reset this CC so writing ~C1, 0 afterwards isn't needed. (If you don't care about the notation then hiding the sfz and writing fp of course isn't necessary)
At the start of a fp write sfz and a p, pp or ppp dynamic and hide them (you can write them in the same text separated by a space) . Then write fp and go into the inspector and make Sibelius ignore it (otherwise it will put "f" as dynamic).
Make a crescendo hairpin and put an ending dynamic.
That should do it. You can create satisfying fp crescendos with this, which in my opinion is one of the best idiomatic effects you can write for brass.
I hope this was helpful to someone besides me. :)