Perhaps a more appropriate title for the thread would have been "lack of pump." Lack of blood flow yields ischemia (oxygen-starved tissue and for all intensive purposes...subsequent death of tissue beyond that point) and that would only lend itself to amputation of something (arms, legs, etc...). I am just a fanatic when it comes to descriptive text; take nothing from it.
I am hard-pressed to believe you won't have ANY visible vasodilated blood vessels provided you're lean enough; but you haven't really given me any stats to go by (i.e. - BF%, etc...) so it is a very difficult read. You did state, " never really had great vascularity to begin with, but as I'd leaned down, my brachials started to pop really well and my forearm veins always were popping out when I was lifting; I even had the trap/delt veins showing up some during the heavier/harder workouts." What I can say is a couple of things based on that statement alone:
1. I am unsure you are predisposed to significant increases in vasodilation based on what you describe or that you have ever been "lean enough" for your body type.
2. You did "lean down" - whatever that means as our definitions may vary significantly; but en route to leaning down, did you cut out carbohydrates significantly? You make no mention of diet / peri-workout supplementation or macronutrient ingestion - so this could be as easily solved as modifying this.
3. You suggested heavier/harder lifting; at least theoretically, that is not the primary stimulus for a pump - at least not longer term. As you increase weight (i.e. - get more intense with your workout); the reps fall and blood flow is a bit more of a challenge to predict. You may opt to set up a workout differently than your current design if it is merely a pump you're after:
1) Compound Lift #1 (heavy, most intense by resistance standards - 80-100% 1RM)
2) Compound Lift #2 (heavy, moderate intensity by resistance standards...after all if lift #1 was as intense as you could get; you're not as truly intense on lift #2 - 60-80% 1RM)
3A + 3B) Isolation Superset (lightest intensity by resistance standards BUT reps will increase precipitously, but keeping in mind you are going with multiple sets in a row - 40-60% 1RM)
4) Accessory Lift (an example of what this is would be "rear delts" in say a back session)
* People lift a LOT more than they need to, which can also deplete yourself...but a workout need not have any exagerrated set-up beyond the above (as simple as it's design looks). You have not provided me with what an example of your workout is, so I am kind of guessing on some of this stuff. Apply more info if you would like either a more focused or more complete answer.