Why there is no VSync option in Roblox
Older versions of Roblox had a VSync checkbox in the graphics settings. Current versions do not. Roblox now manages frame delivery internally and does not expose a toggle for it. Searching the in-game settings menu for "VSync" or "vertical sync" will not find one.
The setting still exists — it has moved to your graphics driver, where it applies to Roblox from the outside instead of from inside the game.
What VSync does
Your GPU renders frames as fast as it can. Your monitor redraws the screen at a fixed rate — 60 times a second on a 60Hz monitor, 144 times on a 144Hz monitor. Without VSync, the GPU can hand over a new frame in the middle of a screen redraw. The result is screen tearing: the top of the screen shows one frame and the bottom shows another, split by a visible horizontal line.
VSync holds each finished frame until the monitor is ready for it. Tearing goes away. The trade-off is a small delay between your input and what appears on screen, because frames now wait for the monitor's timing instead of displaying the instant they are ready.
Turn on VSync from NVIDIA Control Panel
RobloxPlayerBeta.exe is at %localappdata%\Roblox\Versions\ — open the newest-dated folder inside to find it.
Turn on VSync from AMD Radeon Software
Turn on VSync from Intel Graphics settings
Open Intel Graphics Command Center from the Start menu. Go to the 3D settings for Roblox and set Vertical Sync to On. Older Intel driver panels list this under Basic or Advanced graphics settings instead.
Should you actually turn it on?
Turn it on if you notice screen tearing, most visible during fast horizontal camera movement in open areas. It costs a small amount of input delay in exchange.
Leave it off for competitive games. In Arsenal, the added delay works against you more than tearing does. For most other Roblox games, the difference is small enough that stopping tearing is worth it.