Spies can easily slip with a little of micromanagement. Moving spies along a base to reveal its location is a bit tricky because dogs are mostly seen on bases. You are lucky if you won't be able to encounter these canines. When you walk your unit, just make it slow but a loner to make sure you don't get caught by tank treads or dogs. However, sometimes you need also your spy to join a group of infantry to make the player not being suspicious. Making your spies go behind big buildings is also helpful.
But if you hate it, for Allies use Stallion because it's invisible. Borillo for Soviets especially when there is a backdoor on the enemy's base. But the best sneaking vehicle is the Driller which the Epsilon use. Use those and you will be able to infiltrate a base in a matter of minutes.