let me think.

You have a direction vector x,y and want to move in that direction, but with a sin wave added?

first you need the current start angle for your sin wave and you must know that for the vector. Say... phi.

Now, starting with phi = 0 and vx=1, vy=-1, the thing moves up,right and will soon move right, then back right, down:

Now we need the flight-direction angle, I guess. Which would be flight direction - phi of sin wave for this vector.

something like:

psi = fmod(atan(vy, vx) - phi + 360), 360)

Now we need to increase phi for the next update:

inc phi, delta_time

and finally calculate a new flight direction, which is:

newpsi = psi + phi

now fly in that direction

vx = speed * cos(newpsi)

vy = speed * sin(newpsi)

Untested, but I think that should work.