If an arteriole leading to a capillary bed is dilated will the pressure in the vein coming out from the bed decrease ? Will the velocity of blood in the vein change ?
All else being constant, velocity of the Venous blood will be reduced.
The Flow-Rate Equation: Q=AV
If we assume that the body will try to maintain a constant Q, then an increase in the A of the Arteriole means a reduced V to compensate and a corresponding drop in Venous fluid velocity.
If we start to assume that things are not constant, then the story becomes more complicated. Since Venous pressure is largely maintained by musculature and other actions, not heart contractions (though the system is essentially closed-loop), an increase in Arteriole flow-rate doesn't always correspond to an increase in Venous flow-rate. When there is an imbalance - either due to mechanical interference or something else - conditions like subcutaneous edemas can occur where the interstitial (between-tissues) fluid builds up enough to cause discomfort or damage.
*Normal Disclaimer: I am not a medical professional. The above us based upon an Undergraduate level of knowledge.