Could I make handheld weapons that take advantage of a vampires weakness to kill it at longer range? For example, you mentioned the "active ingredient" of garlic. Could I take a concentrated form of that and shoot it on a dart with a dart gun? What if the dart were made of silver? Would the vampire be unable to remove the dart?
Could I make a machine like a flashlight or laser pointer that kills vampires by emitting strong UV waves?
On a side note, what are your thought on the plant wolfsbane?
Finally! Someone who wants to kill vampires instead of becoming one! Well, the answer to your questions in general is: Yes - you can create various anti-vampire weapons, but you want to make sure that they will have the desired effect. Otherwise, you'll just make the vampire mad - and that is not recommended.
Let's start with the UV laser or light. The challenge would be trying to get a wide enough beam that maintained its strength. Most UV lasers in science labs, for example, would certainly be strong enough to burn a vampire good, but the average diameter of these lasers are only about 1 millimeter, meaning you'd have to hit the vampire fairly strategically (like in the eyes or something) to have any real stopping power. On the contrary, a wide spread on a UV light (such as a black light) lessens its power considerably, meaning it would probably do a decent job deterring a vampire, but would take a while to burn considerably. If you have a vampire cornered, you should be able to pull this off just fine, but trying to hit one in motion would be considerably more difficult.
Using the concentrated active ingredient in garlic (allicin) could certainly have a poisonous effect on a vampire if the dose was high enough. Getting a highly concentrated dose into a vampire's bloodstream would cause major sickness, just like when a human gets poisoned. A strong enough dose could kill the bacteria in a vampire's bloodstream and effectively kill them.
You mentioned wolfsbane, which for those who aren't familiar is a species of the aconitum plant belonging to the buttercup family along with monkshood. Essentially, wolfsbane is to a werewolf what garlic is to a vampire. The actual effects are a bit different, of course, but they both work as natural deterrents for their respective monsters.
To be specific, wolfsbane contains a poisonous alkaloid called pseudaconitine, which can be extremely toxic. Werewolves are said to be particularly sensitive to the smell of wolfsbane, similar to how vampires are sensitive to the smell of garlic. Many animals have these built-in instincts to protect them from potentially dangerous and/or deadly plants. It sometimes shows up in vampire mythology that vampires also do not like the smell of wolfsbane, but there is little reason to believe that this is true. Even though this idea was given wide popularity in the 1931 movie "Dracula", vampires are mostly immune from poisons such as pseudaconitine, so there isn't any reason why they would want to stay away from wolfsbane, unless they have a particular problem with the smell. Interestingly, wolfsbane is also highly toxic to humans, while garlic is not.
Finally, anything made of silver is going to be pretty effective against vampires, but again you'll want to make sure you either hit the heart or eyes or some other vital organ (to a vampire). You could hit a vampire with a silver dart, but chances are they could either have someone else remove it or they could use a glove or fabric over their hand to take it out. Silver makes a much better containment material than it does a direct weapon, so if you're going to use it, make sure you get a vampire pinned down with silver instead of just hit with it for a sure-kill.