Vyvanse (lisdexamfetamine) states it's not effected by food since it's broken down in the small intestine. Is it's efficacy changed by urine pH or frequent urination?

Efficacy is not. Affected but excretion of amphetamine is after covalent bond between Dextroamphetamine and l-lysine is broken. The percent of dose excreted as unchanged drug varies greatly depending upon urinary ph & urine flow. At low ph as much as 70% of the dose may be excreted as amphetamine in 24 h, whereas as little as 1% may be excreted within 24 h at alkaline ph (j. Pharm. Pharmacol. 21:241-258 (1969).