im not sure why you have enough gold to buy a null magic mantle, they raised the cost a while ago, and unless youre running the extra 50 gold in utility i dont think you can purchase it...and if you are putting that many points in the utility tree that may be part of the problem right there. As far as dealing with poke, you either have to match their poke or start dodging some skill shots. Boots and pots can help with dodging skill shots.
Some champions that may make things a little easier for you:
Renekton: Insane butter like dueling potential. Its very hard to out trade renekton lvl 3. Dash in, auto, stun, auto, Q, dash out - almost no retaliation, plus a large heal on his q as you leave. )0 retaliation trade but less damage - Dash, stun, Q, Dash out. Even if you are behind you can even things out this way, it has very little counter play.
Jayce: Will win the poke game. Ranged auto and good burst damage with all in potential at level 3 if they are squishy mage. Land an acceleration gate shock blast combo, hypercharge them, then hammer form skyfall spam all you got knock um back and if they arent dead, they wont want to get real close for cs anymore, and you now have lane control, just watch your brush for ganks, an aggressive jayce is a gank magnet.
Also, consider playing some of these mages you dont like in top lane. Lissandra is a hell of a pick in mid or top. You can easily build her quite tanky for a mage. Rod of ages, zhonya, abyssal sceptar, liandry's, all these are good picks - if you are in top lane, you typically arent expected to put out a ton of damage so you can go with the slightly less impactful damaging items that make you really tough and still have a huge impact in team fights, she is a CC machine and thats what everybody wants out of you, and this can be done without a void staff deathcap rush in your item build. You will probably still want the void staff, and deathcap is never "the wrong item" for a final build, but it can easily wait. liandries plus scorc boots gives you 30 penetration and you can ride that out for a long time.
Same goes for rumble, build tanky ap items, no mana, CC machine, play safe, cs, poke.
You can still play vlad. Dont expect to get a lot of solo kills. just be the annoying, forever in top lane, ungankable jack ass that vlad excels at. If you play this way, saving your pool for ganks and never in lane(which is key) your laner will get annoyed and play stupid and thats when you will get your kill.
When being ganked dont panic pool, just run your but off toward your tower and save it for hard CC, like bindings, stuns, and roots or until you are just a pool away from your tower...or just as long as possible. Take ghost instead of flash for more security, your ghost will speed you along while your pooled to get you safely away and can be activated while in the pool.
hope i helped
and 
) I dont like the ap top laners (apart from vlad who is probably my favorite but was apperently nerfed into the ground so i was told) so i don't use them which puts me in this spot. If I use a tanky top there is no item that can defend against them since doran's shield is more for ad resistance(8 auto damage doesn't mean shit to an ability based champion). I could start with a null-magic mantle but it only leaves one health pot for regen which one ability usually out damages everything everything 3 of them would give + champions like 
and 
),restore mana like 
and potions for sustain (only on mana champions)
and potions, helps dodge skillshots
sustain and damage, if they are a poke champion they likely won't survive an engage
helps more than you think, without their full combo they will need to rely on some auto attacks in the very early stages
and 
are your friends
?